According EA first MMA game and THQ's last UFC game MMA isn't selling the way it should. If MMA was what everyone is saying sales will show this with the logic people are using.
It boils down to how deep and realistic a game is. It's not the popularity. If popularity of a sport dictates sells EA's NBA LIVE would have been on shelves right now selling well. NBA 2K sells huge numbers because it's the closest you can get to a sports game out there.