Modern projectile weapons need oxygen.

Soo - it could simply depend on the location and type of atmosphere (if any). You can't fire a gun on the moon - so where does that leave you? I guess you could throw rocks.. probably quite far with the low gravity, but you'd probably have to get close to finish it off.

Then to move to the fiction part - you could easily invent an energy-based weapon and whatever limitations you want to go with it - and a shielding technology that is effective against those energy weapons (but not physical attacks) - leaving you back to close combat (or throwing rocks!). but it would have to be in slow-motion. All movement on the moon is slow-motion :D

Oh, I didn't know that (obviously)

How about an irregular-density atmosphere? It would reek havoc on ranged weapons, but not so much for melee weapons. Especially if the different densities swirled and moved like air currents. You could use guns, but it would be like shooting into a horizontally-moving pool of water.

You also may consider having charged particles in the atmosphere which would dissipate concentrated light guns. Perhaps a laser cannon would dissipate into branches of lightning before hitting the ground.

In the bayonet charges mentioned, the soldiers had run out or run low on ammo and so decided to launch an attack. As long as there are guns/ranged weapons, ammo will be a problem and you can run out. Better have a plan B when it does.

An enemy with an overwhelming melee presence is probably one of the better bets when combined with individuals capable of matching said enemy in hand to hand.

The korean webcomic Knight Run is a good example of this, where the military is relevant to a very large extent, but where the enemy has developed units that can shield, evade or shrug off ranged weaponry. The response to this are the Knights which are specially trained, often modified people with specialist melee weapons that can dissipate the protective field that keeps the monsters from being shoot-able.

http://www.mangareader.net/knight-run

Likewise, Warhammer 40k has a similar philosophy. It's not so much the lost technology that forces them to rely on melee, but rather melee being a part of your gear due to the manner in which wars are fought. (Worth noting is that even Kasrkin, elite special forces of normal humans that would make even our most elite operatives look like playschool children, use knives and bayonettes rather than chainswords and powerfists).

The need for melee comes from all the different breeds of Xenos and Daemons, there are daemons who can be summoned pretty much straight into your face, who are capable of tearing men in half with their bare hands and wield seven foot swords that sear through armour. You're gonna want SOMETHING to deal with them.

Likewise, Orks have immense muscular density, their weapons are huge slabs of sharp metal, they charge headfirst toward combat, and if the horde is large enough, they will get there, simply because they take about ten times as much fire to put down as an ordinary human. Then there are the fanatics, the Khorne berserkers, the Eldar Harlequins (and all of their cast-specific special forces tbh). There are even jump packs, and those are NOT used for positioning other than "Chainsword to face".

The Tau don't have their own specialised melee units, their bodies are too frail, much like normal humans of the Imperial Guard. But, like the Imperial Guard, the Tau are diverse. The Imperial Guard recruit from many different worlds for all purposes, there are deathworlds that breed survivalist melee monsters of men and women. The Tau, they conquer, and they indoctrinate. The Tau armies include units of Kroot, indoctrinated humans, and whatever other xenos they can find. The Kroot, in turn, are fierce tribes of savages that have huge bird-like, non-flying monsters at their disposal. Think of the Kroot like fearless sand-people with rancors.

In a universe where everything out there can tear your face off, you want something to help prohibit that. Shooting only takes you so far when the enemy is made up of an endless swarm of Tyranids (about a thousand times worse than the bugs of Starship Troopers),a horde of Orks hellbent on having a good brawl, or Eldar ninja-like fiends that either teleport or dodge all over the place until they slice you in half.

Imperial guard actually preffer long-range weapons, because if you can't absolutely stop them before they start coming closer than far mid-range, you're probably dead.

The Space Marines have the bodies to go toe to toe with the nasty things out there, with some of the nastiest even, and to further help empower them, there exist the "human" melee weapons.

There's an entire mini-subgenre known as "Swords and Spaceships" where technology on the planets has regressed to pre-gunpowder levels but a number of spaceships have survived and remain in occasional use. This page has an interesting excerpt near the bottom:

http://www.projectrho.com/rocket/futurehistory.php

Hello
In the Edgar Rice Burroughs, John Carter/Barsoom books. The Martians had an honor code,that even the villians
adhered to... "mostly". If a person faced you with a sword,you had to face them with a sword or a lesser weapon,like a shortsword or dagger.Projectile weapons were only used if someone was shooting at you.

As long as at fits in with the rest of the world then any thing can be used.
maybe both sides lack guns or have a cargo that will go bang ie gas
maybe one of both sides has armor.

if you just stop hand held lazer weapons then you are left with bullets and rockets both can be stoped by the armor of today, so a in sci fi world you can just have nano steel armor or some othr sci fi armor.

There are dogs in today's combat that use their teeth and often have their fangs replaced, these are the prototypes of my close combat in the future.

I think many other ideas have to do with the Asian experience being exported. The Chinese military for example tried bullet-proof rattan shields in the Imjin War and overall the militaries of East Asia were better prepared for close combat when their markets were opened for opium, only that this ability was to little avail. I'm still convinced that the current path of military development makes it unlikely to protect against ranged weapons with anything but stealth and faster relocation than targeting movement, while the longing for close combat has much to do with kung fu visual influence and melodramatics.