"War Machine's inclusion in
Marvel vs. Capcom: Clash of Super Heroes
was due to Iron Man being off-limits as he was being featured in
Iron Man and X-O Manowar in Heavy Metal
, a video game published by Acclaim Entertainment."
So Iron Man was going to be one of the just 7 Marvel fighters in the first MvC but they couldn't use him because someone else had the rights and were making a game with Iron Man in it.
I don't think this happens to D-list superheroes very often