Injustice 2 will outsell Marvel because NRS will create a game that appeals to a large number of people. Though you might not enjoy it, the gameplay in NRS's fighters are great. But unlike in, say, SFV, NRS offer more than just core mechanics to appeal to those beyond the FGC. They just make better games.Which only makes my earlier point more painful and valid. Injustice 2 will sell more for all the wrong reasons. In a fighting game you'd think great gameplay would be the one thing that moves units. SFV despite how good a game it actually is proves otherwise.
MKX is a much, much better game than SFV, and deserved to outsell it.