I might have played the game a lot more (and UMvC3 in general, for that matter) if the game had cross-play. Since it does not, it goes untouched. SFxTK has gotten the vast majority of my Vita fighting game play time. DOA5+ did for a bit, but with its online matching issues and then DOA5 just getting killed off by the release of Ultimate, it's also pretty much just sitting there.
MvC is my favorite fighting game gameplay wise, so that's probably why I was able to stick to it I guess. I also had a ton of friends that played in lobbies on a daily basis so I had plenty of competition at the time. I can understand why someone wouldn't care to put the time into it though. That's pretty much how I feel about Injustice atm since it lacks cross-play. I can't see myself having the motivation to spend time learning it if I couldn't compete with most other players. I had like 200 hours logged in marvel last time I checked awhile ago, with about 40 in SFxT before I got bored. I should note I don't have a PS3 though, so I didn't have an alternative (although I'm not sure I could play a fighting game on any other dpad now)