I don't think it's simply a question of some people not being able to handle fame. Another factor is that in our society (at least the North American part), in order for someone to have a successful career as a musician, it really helps to be self-centred, intolerant, etc. A likeable person who is a successful career musician has become so *in spite of* society's expectations -- a doubly great achievement.

I've never tried to be a career musician myself, so this isn't sour grapes...