I agree with both of the posts above me, mainly Lady since I can relate to her.
I join mapless clans from time to time, just for a few hours of RP before everyone goes offline and is never on again.
Some of them are literate, while others aren't.
It's all about what you're looking for.
Anyways, I made CoDM (Cats of Dragon Mountain) ages ago, and the idea was that the cats lived next to, you guessed it,
a mountain filled with dragons.
Every other moon, apprentices would climb up with their mentors and get a scale they could use to morph into a dragon.
I loved it, and so did all my members, until I stared getting hate whispers while recruiting.
Things like "OMG You're so unrealistic, you have dragons.", "You're such an illiterate noob.", and my favorite, "Go kill yourself you illiterate moron."
Yes, those where told to me.
I didn't know how to report then, so.... ;~;
Anyways, a lot of people rage at those people who take a leap outside the box, in any type of RP and not just Warriors.
As for the power-playing, those are people who want attention, or to seem cool.
Trust me, this happens EVERYWHERE, including IRL.
It's a shame to see so many creative, intelligent, or interesting WC RP's fall down due to hate whispers or rumors of stupidity in the group.
The stereotype for WC is very wrong, but yet sadly, it can be true.
The reasons I see:
1.- As you stated, overpopulation.
2.- Plot repetition. One of my least favorite things about warrior cats in general.
3.- Immature kids, as you stated.
4.- Power-playing folks. A vast majority of warrior cat roleplayers powerplay immensely.
5.- 'All-that' characters. Characters that are strong, brave, all-mighty, 18 moons because heck, they're awesome, etc.
6.- Map-claiming.
Example:
Wolf[1]: Hey, why don't we have our roleplay here? ^^))
Bramblestar: NO! This is my Clan's territory and you can't come here.
Wolf[1]: We won't interfere with your roleplay sir, we will keep to ourselves..))
Bramblestar: Seriously there's a whole map just leave.
Etc, etc.
Those are just some reasons I think. I also agree with previous posts to some extent.