Really, I would like to believe Capcom, as the earthquake was tragic, but sadly, I don't.
Capcom has locations in Tokyo and Osaka. The effects of the earthquake on Tokyo were relatively minor, and Osaka was not affected at all. The rolling blackouts due to the shutdowns of the nuclear facilities may have slowed progress, but that doesn't make a single bit of difference when it comes to releasing DLC vs. putting the content on a disc. My guess is it's really just Capcom being Capcom, who just seem to offer little to no DLC or digital distribution for their games to begin with, instead opting to release everything on physical media. They even did this for Street Fighter 4! They just knew that the fans wouldn't put up with it as well this time, so they offered this excuse. Again, I can see how the process would be slowed by the earthquake and tsunami, but not how it would warrant a disc instead of DLC.
Though if any Capcom employees were affected by the disasters, my sympathies and support go out to them.