Most of the RPs in RPB die around or before 10 pages. I've seen noob RPs which have lasted longer than the pro ones.

Generally because lack of activity rather then lack of interest. Good RPs that have several paragraphs a post will probably only get a few posts a week, especially when it's compiling a week's worth of one-liners into one good post.

Or those who are participating blame writer's block (a fictional disease which afflicts those who write).

reminds me of Hakker during DoL, always mentioning his continual writer's block.

Writer's block is overcome by, funnily enough, writing.

Um, guys? Getting a little off topic here.

Oh right, sorry about that. Did I mention that your RP is kinda plain to me?

Yeah, I know. I didn't actually want to make an LP to begin with, but most of the LP topics usually:
A. were dead
B. I wasn't interested in
C. had WAAAAAAAAAY better RPers then me, so I didn't want to embarrass myself
D. had been ruined by some noob or spammer
So, I just gave up looking and decided to try my hand at making my own. It was rushed, so I'm trying to think of things that would make it better.
To sum all that up:
Couldn't find a good RP
Got fed up.
Tried to make my own.
Couldn't bother my lazy @$$ to spend more then 2 seconds on it.
Pathetically attempting to make it better.
I sure am happy, aren't I?

its because the pro's know when they have lost, and the noobs will drag it on because they all think that their charecter is invincible and that they deserve to win.