its probably because fox owns the movie licenses to x-men... which is why they'd rather have the mutants job to the avengers even if the mutants may historically be more popular
and i never liked how they handled the whole hulk situation. it basically got thrown to the bushes as soon as WWH ended. the very next day there's a red hulk and so hulk turning himself in following WWH and being held in voluntary captivity doesn't mean anything. him makin his comeback has no shock value either cuz there's already 'been' a hulk there this whole time and there's no grand return to shake the marvel U because hulk starts off doin some cosmic cartoony beef bullshyt with the red hulk and silver surfer or something.
then they have some weird as story where there's about a dozen different hulks running around or something... but that event was going on around the same time that either dark reign or seige was going on... so it basically wasn't being acknowledged by the greater marvel universe/community. Which is funny because 1 hulk alone was able to main event the marvel universe but upon his return, he basically became just another random character who had absurd stories that didn't mean shyt outside of his own book. Seriously, following WWH, green hulk should have been kept in the sidelines until Seige, in which he should've been awoken/freed/convinced to come out so he could take down Sentry. That should have been Hulk's comeback W... not Thor's.
it would have re-established Hulk as the strongest there is, the most dangerous and unstable muthafukka around, and also its greatest hero that day --who still doesn't want shyt to do with any of the cacs who sent him to out of space