I don't know what to think of Kanye. Some claim he's a genius. I have no evidence to the contrary, but his public demeanor makes for a difficult-to-overcome level of bias.
I think there's a polarization effect in calling someone a genius. Maybe we should lay off with the big lingo and then there might not be so much backlash. If we just said, 'he's good at what he does' would people care so much? Would we feel the need to define him in such hyperbolic terms?
As for David Crosby, I don't know what his game is. I guess he just thinks that Kanye sucks, which if you ask me, is as valid an opinion as thinking he's some sort of musical deity. And let us not forget, Kanye himself thinks he is some sort of musical deity. If you said, 'musical deity' to him, he'd be all like, 'I dig that. I am that.'
Short Answer: Maybe it's just jealously because everyone on this planet and their dog wants to fuck his wife. David Crosby's wife, I mean.