According to a report over on Kotaku, Netherrealm Studios' Mortal Kombat X seem to be coming very late to the party with its old-gen ports. First the versions for PlayStation 3 and Xbox 360 were supposed to be released when the game launched on PC, PlayStation 4 and Xbox One back in April.
According to the site, retailer Gamestop was told that the old-gen launch has been moved from "summer" to "fall". We don't know the reason for this, but with so much time and attention being directed at patching the PC port, it might be that they have their collective plate full at this moment in time.