[quote name='Jucifer']Disney actually owns Marvel now, which could explain the problems with the DLC.[/QUOTE]
Maybe, maybe not. It's hard to say since no one outside of the two parties knows the facts. The way I see it, both Disney and Activision are at fault, but mostly Activision. I know that Disney prefers to keep all development in house whenever possible, but that doesn't mean it's impossible for outside studios to license their game. We got the Sega's Iron Man 2 coming out in May and Activision's Spider-Man: Shattered Dimensions in September. DLC licenses are renewed anually (at least with Marvel they always have been) and Activision's license expired January 1st 2010. Disney probably made it really expensive for Activision and/or Activision decided not to renew their license because the game was such a flop. It's highly unlikely that they never got a chance to renew, since there were rumours about this before the content was even removed. After all, we still have Capcom's Marvel vs. Capcom 2 on the marketplace, which is technically DLC.
It's probably easier to point fingers at Activision because they're the only ones who have gone on record to discuss this. We'll probably never know the full story.
But if there are still people who are interested in MUA2, it's still possible to get the DLC if you have a friend with the content. As far as I know, one can still download the content from their download history, therefore a friend could transfer the license to you so that both can use it (but the owner will only be able to use it when they're online).