I think, often, it's not bad players that ruin things it's bad people. Over the last year and a half or so I've been RPing almost exclusively with people who are new to the hobby. Their RP was, by any standard on this board I'm sure, bad. That doesn't mean the experience was bad and, over time, they got better. Too many times I've seen new players on games chased off because they were "bad RPers".
I find most new players are very open to constructive criticism. That is generally not what they encounter. They get ostracized, mocked, and ridiculed right off of games. This can be particularly true of someone trying to stretch their RP wings by trying to play something against what they are in real life.
Maybe that dude playing a gay man is a bit over the top. Sure. Maybe it's because he's playing the character as he's seen it portrayed on various TV shows or movies. Or maybe he's just inexperienced. Maybe he's just an asshole trying to portray a caricature. I think the real problem is most people on MUs jump right to the last option and lash out at the player.
So when we ask "why don't we see more diversity on games?" or "why don't we see more new people entering the hobby" I think it boils down to this right here. There are people who will come to this hobby to explore RP. I've seen it. I've seen people new to RPGs in general excited to be able to RP online. I've seen people come from MMO RP servers excited to be able to stretch their wings textually in ways they can't in the MMO. I've also seen these players chased right off of games because they did something that set off someone's "radar" and made them suspect that the new player was "this" or "that" or "whatever the hell else".
TL;DR Version: Fewer new people try new things because they get burned for it by jaded RPers who assume the worst from every experience they have.