A Variety of reasons. 1.) No big threads' plots seem to interest me.
2.) I'm too lazy to make a new character (So I'm pretty much left with Q for everything. Though I do have other characters, they hardly ever fit in any plots ant aren't as versatile or fun to play as Q).
3.) Most threads these days have randomass requirements that are RETARDED. Full character bios no one's going to read, "real" images (no anime), ridiculously obnoxious post styles, etc. It's stupid and I want no part in that shit. An RP is about story and writing, not how pretty your damn post is. (That and the full character bios are just something I'm too lazy to do)
4.) No freeform RP anymore. Freeform is where you basically let the RP write itself and you don't post bios or plan anything. You just have an opening post and let it go. This is how I grew up RPing. Everything now is so structured and planned out... There's no freedom anymore.
5.) Big threads either move too fast or too slow. I'm afraid they'll move too fast for me, so I avoid them. But if I'm on a lot, then the others aren't on enough (or it gets stuck on one person's turn) I'm stuck until they decide to post. Which is lame.
6.) 1x1s offer leeway I and freedoms I can't do in group play. I can bend rules and sometimes auto-hit and I can PM the person to bug them about certain issues. While you can do this in a group rp too, it's generally just harder and if you're really into it, it would've been more beneficial to have a 1x1 with that person anyway (done it before).

Though above all, I really think its 3 & 4 that are holding me back the most.
I'm not abject to larger RPs. I actually kind of miss them.
But I just either never fit in or I don't have enough time to give my attention to it. I mean, if you miss a day or two and it's a fairly active thread... Depending on how many people are rping there, that's a LOT of posts you have to read. And it's not like rp posts are that short these days. I'd feel bad if I skimmed or skipped.