Capcom says that March's tsunami was partly responsible for the change of approach.
The extra content in Ultimate Marvel vs Capcom 3 was originally slated to be released as DLC, Capcom has said, but the March earthquake and tsunami disaster led to the publisher changing its plans.
Staff health issues also played a role in the publisher's decision, according to the latest issue of Famitsu (via Andriasang).
It's also been revealed the new expanded game - which is to be released at a "mid-range price" - is scheduled to get its own DLC at some point in the future. One proposed mode allows players to use an infinite supply of Hyper Combos, which just sounds chaotic.
The 12 characters to be added to Ultimate Marvel vs Capcom 3 include Frank West, Rocket Raccoon (still baffled on that one) and Phoenix Wright.
Andriasang doesn't give a full translation of the Famitsu article, but I suspect there was a bit more explanation given than, "Oh, the tsunami made us change our minds".