Possibly because for MUA1
I think they're trying to avoid people hacking the game, and creating lots of other Marvel character skins for themselves. As this was done for some lucky folk who then got to play it using other characters like Punisher, Namor, Dazzler etc, etc. As well as quite frankly better designed versions of the game's playable characters. (The Hulk in particular looked much more like his comic counterparts..)
Guess they're a bit touchy about other people improving their game.. (Shrugs..):(