Some other answers suggest adding humidity to prevent combustion. This is a terrible idea, as has been stated. However, it could work for a different reason, especially if we add some sand.
Firearms can be unreliable
Or, at least, the modern version, with high effective ranges and fancy concepts such as "semi-automation". More simple weapons aren't quite, though.
The most basic version of a gun (as we think of it) is as follows:
- Take a projectile.
- Stick it in a fireproof tube.
- Make an explosion that launches the projectile out of the tube at a high velocity.
This can be an improvement over bows and the likes at very short ranges, if your goal is piercing armor, but it's very unreliable at longer ranges. Guns of this sort have been around since around the 1100s, and the basic gist of it wasn't drastically improved upon until around the 1850s or so. They were mostly used for armor-piercing purposes and due to needing much, much, much less training to use effectively than a bow at similar ranges. However, they were exceedingly inaccurate and took long to load.
These would still be viable, but only due to simplicity... and even then, only to an extent. Though, I suppose the title of this subsection is somewhat innacurate...
MODERN firearms can be unreliable.
One of the more popular handguns in pop culture is the Desert Eagle. While it may look cool and sound nice, it's really a terrible weapon. It's incredibly heavy (making it hard to hold steady for aiming), and jams exceedingly frequently, especially in humid or sandy areas (ironically making it useless in deserts). Despite all that, it's also WAY more expensive than most other guns (just under $2000 USD) - you can buy actual, military-grade, melt-through-steel-in-an-instant laser weapons for around that price.
Most firearms are, to an extent, similar, anymore.
Guns, these days, are astoundingly intricate. If I tell you to go make a .44 Magnum, you wouldn't be able to. If I went to someone who regularly machines things as a hobby and told them to make one, they wouldn't be able to either. Only a huge assembly process specifically created for the production of .44 Magnums would be able to make one with any efficiency, because there's just so many tiny parts that all need to be perfect to even get off one shot.
As such, even if I already have an assembled, working .44 Magnum, it can still have issues firing if it gets full of sand or starts to rust or the likes. And a good, old-fashioned revolver is one of the most reliable weapons you can own - just imagine what it's like for even more complex weapons with 35 extra parts for feeding an ammo belt and another 12 for ejecting spent cartridges, 17 for feeding in new bullets from a clip, et cetera.
Let's just get to the point already
Now imagine some incredibly-humid, sandy-as-all-getout land where so much action goes on that you'd never be able to sit down and do maintenance even half as often as you should under good conditions. Within ten years, nearly all complex firearms would be so rusted and ruined that they could never fire a single bullet again.
Now, as for explosives...
There's just no stopping explosives
A bomb is even more simple than a gun.
- Get something that can explode
- There is no step two
People make bombs with a quarter-cup of laundry detergent, an empty milk jug, a piece of flint and some metal to hit it against. These are effective enough to be as costly to troops fighting in a war than the actual "fighting in a war" bit. Making the air more wet isn't going to stop it. Making the air amplify that explosion just makes it an even MORE attractive option! There's just no stopping bombs without deoxygenating the atmosphere. Sorry.