I've not seen Sean credits, I've seen 2 Cons credits on the two weakest songs on 808s (and the songs he's on, on kanye two first albums). I've seen Rhymefest writing credits on what could be my two, as far as lyrics goes, favorite Kanye West songs (Gorgeous and Jesus Walks).
Kanye obviously didn't send a gift cause that nikka was out dissing him and Good Music on radio.
I don't know about that shyt, too personal for one to know the real reason but the simple assumption is his loud mouth. Kanye is the boss, don't ever expect shyt from the boss specially if you disrespect him and you're highly replaceable.
He should be thankful, what happen to humble cats. He loved Kanye when he was rolling high, in the studio with all the big players, fukking mad bytches, even featured on Kanyes albums (twice) unlike a successful cat like Big Sean. He never sold shyt, never got any buzz even with the biggest rapper putting him on songs. His other man Q-tip let him hang out with the legendary ATCQ for their comeback album on 5 fukking songs and he rolls out and goes at these dudes like they didn't do everything for him. Acts like he deserves what the have and shyt

.
He's around too many yes men, someone need to tell him that he's a good rapper but good rapper doesn't mean shyt if you can't attract fans, even with Kanye, A Tribe Called Quest, Common Sense and more buzzing you up. He's with soft cats, he needs someone like Jay-z to tell him to sell weight, go to school, get a buzz or shut fukk up before you get blackballed to death. If Kanye and Q-tip put me on their shyt when I couldn't amount to shyt on my own, I would just salute them and understand that I was dead weight.