Why isnt Hulk part of Iron Man 2 after Stark appeared in Hulk if Iron Man made 1st?

Stark appears in Hulk 2, just to give us a hint that hulk and Ironman will join together in fighting evils in future. So this is all about the movie Avengers, which is being filmed. Not only Ironman and Hulk, we have more interesting characters in this movie to keep us in awe.