Does Marvel really need an Injustice of their own when they’ve been involved with the Marvel vs. Capcom series since its origins in 1994? That iconic crossover franchise’s legacy is what inspired Mortal Kombat vs DC Universe which led to Injustice in the first place.
The MvC series has kept a strong presence for a long time at EVO tournaments and for casual players as well. People aren’t going to get tired of seeing Marvel characters fighting each other in video games, especially when they’re also paired up with and against Capcom characters.
EA tried to make a fighting franchise of their own with Marvel characters with Marvel Nemesis, but that flopped for other reasons.