because they've got the Hulk in it and he didn't stick around?
More likely, they've got the Hulk in it because he's already had two movies, and they're combining all the characters from their movies franchises: Cap, Iron Man, Hulk, Thor, Black Widow.
If they had the movie rights to Spidey, he'd probably be an Avenger, too!
Besides, Cap and Black Widow weren't in Avengers #1
, so it's not like they're too worried about staying faithful to that one old issue.
Captain Craig: they don't need to just cherrypick stories involving the Hulk. They can use any old story and just stick the Hulk in it, the same way they're stick Cap and Natasha into whatever origin story they choose to tell.