Has nothing to do with Sony, bro. Activision pulled it. Very shady business involved. Since the game didn't sell nearly as well as the first game Activision decided to pull the content to avoid crediting the developer studio, Vicarious Visions and to cut payments (quotas needed to get DLC2 underway). More DLC characters were planned but once sales figures came in it went up in smoke. Only the compatibility patches are available, which is pointless because nobody plays it online these days anyway - not that many people were playing it online when it was brand new. The character additions were awesome though, and for folks that bought the content back then they can still download it from their download history list.
Once things went south with the game Activision put a 'limited time' sticker on the DLC releases, stating it was 'always intended to be a limited time DLC offer', and they were supposed to pull it in December of that year, but decided not to wait and pulled it a month earlier than that. Due to complaints they decided to temporarily rehost the content a year later for a few months (if that long), and pulled it for the last time.
Currently no plans to make MUA3, and likewise another X-Men Legends game. X-Men Destiny was only made to keep the license from expiring, and at that wasn't a very good game at all. Spiderman gets all the attention as the Spidey games continue to sell, despite Web of Shadows.
