why does falling prone make ranged attacks against you get disadvantage?

Because you are presenting less of a profile for your opponent to attack.

Because people lying on the ground are smaller targets and thus harder to hit.

As the others have said, i just wish the rules also accounted for shooting at a prone target within 5ft, that's a bigger target not smaller.

Making a ranged attack at an adjacent opponent (within 5') already imposes Disadvantage. Being that close to the point of origin of the projectile makes it easier to dodge.

Quote from TheCubester >>

Making a ranged attack at an adjacent opponent (within 5') already imposes Disadvantage. Being that close to the point of origin of the projectile makes it easier to dodge.

Hard to doge if you're prone. :)

But yeah, simple rule is that either due to prone or due to being at point blank range, you have disadvantage on the shot.

If standing yes, if someone is standing over you whilst you're prone, it's a lot harder to dodge that. What I'd like is an exception to the rule that allows advantage or at least counters the disadvantage of traditional ranged in melee.

Prone doesn't mean you're off your guard. You can still roll around and dodge a shot.

Attacks within 5ft have advantage and the printed creature has disadvantage on attacks, there is clearly mechanical disadvantage there. Rolling to dodge isn't as easy as stepping or altering your body whilst standing either, especially when wearing armor etc. 

If you are easier to hit with a melee weapon why wouldn't you be easier to hit with a crossbow?

The Advantage for attacking a Prone character is cancelled out by the Disadvantage for attack with a Ranged attack within 5'.

So it's a wash.

I would also point out that the Disadvantage for attacking with a ranged weapon while within 5ft of an enemy isn't because the enemy is harder to hit - it's because that enemy is harrying you - preventing you from aiming properly.

You'll note attacking anything with an enemy in melee while you have a ranged weapon is Disadvantage - not just the enemy in melee range.

I also feel that hitting a prone person at 5' with a sword is a whole lot easier than trying to shoot them with a hand crossbow. So I'd say disadvantage is totally fine.

If you had the crossbow expert feat, would you then have advantage on the shot of a prone creature 5ft away?

Quote from Utticer >>

If you had the crossbow expert feat, would you then have advantage on the shot of a prone creature 5ft away?

Yes.

Without it you have both Advantage and Disadvantage (and therefore neither) - but with Crossbow Expert it removes the Disadvantage so you have Advantage.

[never mind]

Yeah that always gets me too, I always remember the rule being "prone gives disadvantage to ranged attacks!" but in reality it's "prone gives disadvantage to attacks beyond 5 feet from the creature." So with Crossbow Expert you have no disadvantage for making a ranged attack in melee, and are just left having advantage against a prone target that you are standing next to. And without it, you're making a normal attack, not one with disadvantage. 

Quote from Chicken_Champ >>

Yeah that always gets me too, I always remember the rule being "prone gives disadvantage to ranged attacks!" but in reality it's "prone gives disadvantage to attacks beyond 5 feet from the creature."

 Same.

Quote from DorkForge >>

If standing yes, if someone is standing over you whilst you're prone, it's a lot harder to dodge that. What I'd like is an exception to the rule that allows advantage or at least counters the disadvantage of traditional ranged in melee.

 Crossbow expert

That's not an exception or clause in the rules, that's a feat giving you a specific benefit because you've mastered a specific type of weapon. 

It doesn't really address what I was talking about and in a game where your chance to actually get feats (if they're allowed)its far from ideal.

5E ain't a simulation, it's a game system that tries to keep itself pretty streamlined. It doesn't try to calculate wind speed, or glare, or elevation, or humidity, or whether a target is in motion or stationary, or a million other things that probably would impact your ability and accuracy while shooting something. Instead, we just know that generally its harder to shoot at someone laying on the ground than it is someone who is standing up, which is generally true, and that it's easier to shoot a target up close than one that's further away, which is generally true. Good enough, if you want something more complicated than that you're in the wrong game system.