Injustice uses a game engine modeled after Mortal Combat, except it doesn't pull it off as smooth or as well. Mortal Combat was an arcade game before it was a console game, and is already widely popular and well known for being successful, whereas Injustice is brand new and not very well known.
I'm only 24, but I can remember when Mortal Kombat was a major franchise and causing massive controversy to the point of Congress weighing in on the issue. But this franchise is still alive? And people are playing new entries within it still? What appeal could it still possibly have? The gameplay was never that good.