I think all the main players are signed for a specific number of films, and they probably either turned down anything else, or else they might have had to pay out millions more just to get Tony Stark on screen for one short scene. Anyway, since it seems like they are trying to phase him out of the spotlight as the main Avenger and replace him with Dr. Strange, I can see not trying to keep having him pop up.

I'm not sure about Jeremy Renner though. Unless there was scheduling conflict, they should have found a way to get him into at least a scene or two.