A good reason for people and I agree with what
zaf said I remember in one post, is that people feel they are getting "a fresh new" start in Injustice: Gods Among Us because they were ass in MK9 and will continue to be ass in Injustice, and are thinking this game will lead them to some hollywood status if they play it enough to become good. Hence, everyone blames MK9 as "broken" when, most players who played it seriously, did well and know how to deal with the stuff.
Even ChrisG said on FingerCramp stream, "if you suck, you will stay sucking, there's nothing to save you".
I personally still like MK, I think it's a bit better than Injustice: Gods Among Us in terms of it's pacing. I like the game because I like superheroes and I like the unique characters.