X-Men: Byrne blames Claremont.
Fantastic Four: Byrne blames increased editorial interference
Superman: Byrne blames Dick Giordano
Next Men: Byrne blames the comics market
Danger Unlimited: Byrne blames retailers
Avengers West Coast: Byrne blames "a bone-head EiC who did not pay attention."
Captain America: Byrne blames Jim Shooter
Hulk: Byrne blames the Editor in Chief.
Indiana Jones: Byrne blames Jim Shooter and LucasFilm.
She-Hulk: Byrne blames Bobbie Chase.
X-Men (second run): Byrne blames Jim Lee.
This is all from Byrne's FAQ. These are his stated reasons.
As Ceci beautifully illustrates here, you rarely have to go beyond Byrne's own words to say to yourself, "Geez! This guy sure quits a lot of jobs!"
Even if I'd never heard of Byrne's reputation, just reading through his own FAQ section would trigger the ol' spider-sense that this guy's stories sound just a little too similar and a little too loaded.