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It's a game.

Welcome to most glorious event of the year! As you all know, each year we pick a planet and select an area of specimen to defeat. This year we've chosen a fun classic planet near Sol, which they've named 'Earth'.

These creatures are about our height and have a weird pale uniform color. They fight using physical energy. They have long metal sticks called swords. Your challenge this year is to defeat them at their own game!

What you seem to be describing is some form of 'gap' in their technology tree, assuming the same development path as we have taken.

For us, we consider electro-magnetic technology to be almost child's play so as to be so simple, and a massive amount of our modern technology is actually based on this. But, the only reason that we think that is because of the Maxwell equations of 1861-1862. Prior to that, if someone like Queen Victoria had said 'you engineers, build me a box I can speak into that makes my voice come out in front of all my subjects', it simply couldn't be done because we didn't have the scientific theories available to design a practical application like radio.

We've already made some scientific progress integrating weak nuclear forces with electromagnetism, and we expect the next one will be strong nuclear, then gravity. Once we reach that point, we have a grand unified theory that could give us incredible control over the universe and our place in it. But, that's assuming there even is a GUT to be discovered.

The answer to your question however seems to be in the power sources for your alien ships. If your aliens have mastered theories around Strong Nuclear forces, then they may well be able to initiate fusion reactions, giving them vast amounts of power for spaceships, but not have even basic radio. Also, based on what we know, it's possible that fusion doesn't scale down or miniaturise well, meaning that it's possible (especially if their planet doesn't have the ingredients for gunpowder readily available near the surface) that your aliens could have fast ships, but only basic weapons.

Ultimately, what we think of as the 'obvious' technology tree is really a function of what we've discovered through our own scientific endeavours. But, it's entirely possible that other beings, having discovered other principles of the universe first, could get here and then go 'damn! How come they have such powerful weapons and haven't even mastered the basics of space flight?'

The aliens came here for a peaceful reason. But they had very bad discipline, and some of them decided to invade earth using whatever they had. Swords are too weak for them to be considered a weapon that requires any regulations.

They have better equipment, but not with them.

A spaceship is not a place to bring high-powered weaponry. A stray shot can easily damage critical components or puncture the hull.
Replacement parts may be months or years away.

Consequently, the doctrine is to favour melee weaponry when operating near spacecraft.

Your aliens might be able to manufacture some low-grade ranged weaponry given time but there's only so much the Machine-Shop on their ship can actually produce outside of small tools and replacement parts for the ship.

Magic

If your humans have access to magic, perhaps that could be the answer? The aliens attack a city or village and it is a slaughter as they use guns, bombs, and plasma weapons. Then, at the last moment the towns Elders, a group of wizards or sorcerers, create a powerful spell to deactivate the weapons that have slaughtered them.
Perhaps they could place a curse on them so that any one who uses those weapons is killed, forcing the aliens to use swords and other lesser weapons.

With the weapons deactivated, the attacking aliens are furious but can do nothing as they do not understand the magic and have no control over it. The spell is limited as the wizards were rushed so only affects the weapons, not the transports or ships.

It also strongly signifies that although humanity is weaker, they can still win the war.

This could also affect the plot and story in interesting ways. If the Elders are dead, there is no one left to turn to about using advanced large scale magic (like the curse), increasing the stakes for our heroes.

A potential sub-plot could be the last Elder survived and when they are killed, the curse dies with them. The aliens are now frantically hunting them down to lift the spell/curse.

You mention dragons and magic, so we're talking about a fantasy scenario.

Have your aliens initially invade with their standard plasma/laser/whatever guns blasting people to ashes. Then the humans figure out that the lowest-level, easiest defense magic completely blocks that type of weapon. Maybe aliens use lasers and the first level "minor darkness" spell that every wizard learns in the first semester in magic school and that everyone thinks is mostly useful for playing pranks completely eliminates those weapons. Or the $10 protection rings you can buy on every street corner that are so weak that people use them mostly for not hurting themselves so much when they bump into furniture completely blocks plasma. You get the idea.

Instead of taking the weapon away from the aliens, make it useless. The aliens suffer some pretty horrific losses before they get the idea that if high-tech doesn't work, then low-tech it is, and go and use human melee weapons.

There is the trope that magical energies cause advanced technology to malfunction. In many universes that combine magic and technology (such as the Dresden Files or the old CRPG Arcanum, and to a lesser extent in Shadowrun), magic and technology do not mesh with each other.

In the Dresden Files and Arcanum, simply having a magic user be present near a piece of tech is sufficient for it to start behaving erratically and even break down.

Maybe in your world, even the presence of magic on the planet is enough to cause high tech small arms and ground vehicles to malfunction? The spaceships might work because they have heavy metal shielding to protect from interstellar radiation, but as soon as you try to fire your blaster outside the ship, it blows up in your face.

It's a sport

This alien species is very advanced. As a sport or entertaining activity, they invent a new fashion of hunting, similar to the saga of Predator movies.

The major difference with the Predator movies is that instead of using all their technology to track down and hunt warriors they make something more exciting: fights at equal levels.

A spaceship is sent to the planet, scan down the entire surface and look for an intelligent race. Then, investigate their culture and technological warfare: weapons. After doing that the sport-aliens or hunters are sent to the surface using the same weapons as the intelligent species of the planet use, in our case swords.

The challenger or objective of the activity is to kill the major amount of individuals in the minor amount of time without dying nor using more advanced technology. The alien who kills the most amount of humans is rewarded with honor/trophies/status/money/etc.

It's just a game. Even more, it could be televised in real time!

Can't numbers be a balancing factor? Let's say the spaceship holds a couple thousand fighters, and there are at least millions of humans. That could work well even without magic and dragons. The humans have bows and crossbows, siege weapons (even the romans had catapults and ballistas), so it would be weird if the aliens didn't have any of those (or couldn't copy any of those in a day).

A fairly simple one but it could just be a matter of culture for the aliens. As a highly honour based society they wouldn't dream of anything less than taking their enemies on in personal combat, seeing ranged weaponry as weak of character and cowardly.

They came "to serve man".

Part of the ethos of getting meat for the dish is that certain weapons must be used.

Bloodsport / Rite of passage / Initiation / ... .

Various similar - ie the means of hunting the game is part of the thrill of the chase.

cf not shooting a sitting duck.

Similar is done in a Larry Niven story where large super intelligent (vaguely) dinosaur like creatures live in a high pressure environment. Hunters come to hunt them with well defined allowable weapons. The creatures are bored out of their minds by their environment and welcome their side of the 'hunt to the death' situation.

Searches .... Bandersnatchi on Jinx

• Jinx, orbiting Sirius A, is a massive moon of a gas giant (simply called Primary), stretched by tidal forces into an egg shape, with surface gravity at the habitable areas near the limits of human extended tolerance. The poles lie in vacuum, the equatorial regions are Venus-like (and inhabited only by the Bandersnatchi); the zones between have atmosphere breathable by humans. Jinx's poles become a major in vacuo manufacturing area. Jinxian humans are short and squat, the strongest bipeds in Known Space. But they tend to die early, from heart and circulatory problems. There is a tourist industry which provides substantial useful interplanetary trade credits for the Bandersnatchi, who allow themselves to be hunted by humans under strict protocols.

Bandersnatchi

https://larryniven.fandom.com/wiki/Bandersnatch

https://wiki2.org/en/Bandersnatch_(Known_Space) <- good

The aliens are not willing invaders. They are invading, of course, but only because they have no other choice.

They might be the last remnants of their race, with almost all their knowledge, technologies and resources lost, forgotten or spent, and their invasion attempt is their last resort try at survival. Or at least their best bet to secure a few more days, years or centuries until the inevitable demise of their species.

Or they might be criminals who were condemned to exile to a hostile planet as a form of punishment or exotic execution. The ship has brought them against their will and without any weapons to defend themselves or tools to return home. All the while the law-abiding aliens back at home sit on their couches and watch that fancy new reality show about a rag-tagged band of shipwrecked guys fighting with improvised weapons against a strange bipedal humanoid race - in glorious 3D transmitted by hovering micro-drones.

My initial thought was simply that the ships were not actually theirs (either stolen or borrowed), such that the actual technological level of the invaders was such that swords were the best they had. However you describe them as highly technological so that's out the window. In its place

They aren't engineers, they're technicians

What I mean by this is that whilst someone on their homeworld created their technology, the ones on the ship only know it well enough to maintain and repair. If it was to far damaged from earlier events, or simply didn't function in the environment, they may not be able to work out what was wrong. Imagine an IT support person trying to tank.

Get rid of their spaceships

The only way, other than the aliens choosing to have an even fight, to make this even remotely plausible would be to have them lose most of their tech - at the very least all use of their spaceships - around the time of their landing on earth.

Any spaceship capable of interstellar travel, no matter how limited, is a far more potent weapon than anything a medieval society has access to. If they can get, say, ten people to orbit, they can probably just as easily drop a half-ton rock on your capital. Or crush your armies by firing their trusters once or twice. Or do whatever else they like because swords and arrows aren't much of a threat compared to colliding with space rocks at relativistic velocities.

Even if you ignore a spaceship's offensive uses, the humans would need pretty much unlimited teleportation magic (which is also a weapon potent enough to render swords etc. meaningless), otherwise the aliens' logistical and strategic advantage will make them unbeatable. You can't catch them, you can't siege them, you can't defend your crops, livestock or food stores, or infrastructure.

However, take that away from them and the tides might turn:

They probably (used to) rely on their spaceships for almost everything

Let's be real, a foot soldier, even with the fanciest of guns, is not going to be much use in a space battle. Why would the aliens waste valuable cargo space on infantry weapons when they have a gigawatt laser turret on their vessel? Why lug around explosives when you can level a city by pointing your thrusters at them? Better pack more fuel or better food or that redundant life support system they recommend for long voyages.

The only way you would be worse off is if you ran out of fuel, or broke something critical during the landing, or the engine had some weird interaction with the local atmosphere. But that's never gonna happen, right? Right?

Okay, crap, uhm, let's just fire up the fabricator on emergency power to make ourselves some pointy sticks, yes? And pray that the skiff still works, we're gonna need it.