Where did you get the sales figures? From what I've seen so far it sold as much as a fighting game can reasonably sell at this point. We are at the end of the gen, casuals already bought a fighting game for their console, sometimes more than one, they're not going to buy another one. The fact that TTT2 is heavily focused on multiplayer didn't help, either. I love that approach (tons of content, excellent online, every character unlocked from the start, no resources wasted on retarded storylines) but I understand the package can be underwhelming for some people.
Still, catering to the hardcore has an advantage: if the game is good, it'll have long legs. If TTT2 keeps being streamed and played in tournaments, it'll keep selling.