They die for 4 reasons in my experience:
A) forgetfulness = Someone does not post for a while. And nobody pokes them about it or kicks them out.
B) Directionless = Nobody has anything to do.
C) unavailable = Someone has to go for a while, so it dies.
D) As Zane says, the main person loses enthusiasm.
The strongest RPs generally have less players, and a strong direction they are heading in.