I'd like to see him available, too, but he's a villain. I know that he's no longer a villain in the comics, but that doesn't seem to matter for these adaptations. Once a villain, always a villain. The same way that comic Magneto has been dancing back and forth over the heroic line since the 1980s, rarely going back to "true" villainy anymore, but in most adaptations he's still portrayed as the full-on villain bent on world domination he was in 1960s comics.
... of course, that same rule doesn't apply for other former villains like Wolverine, Wonder Man, Hawkeye, and Black Widow, and especially Deadpool (who is *still* mortally ambiguous at best, yet is presented as a full-on hero in adaptations).