The reality of it is that this game is being released too early in the life cycle of the next-gen consoles that it has to be cross-gen. What is more reality than the fact that it could've been using the UE4, is that people don't switch over completely to next-gen until 3-5 years after release. The PS2 was still being produced for a whopping 7 YEARS after the PS3 was released.
Sure this game would have been amazing and a state of the art technological masterpiece if it was on UE4, but I'll go as far to say that NRS, and WB for that matter, would LOSE money if they released it as next-gen exclusive at this time, considering the amount of resources that will have to go into utilizing the top of the line next-gen technology to develop this game.
You guys need to see the picture for what it is presently. Yes, we're all MK die hards, but think in the perspective of a casual fan. In no way is MK at a Grand Theft Auto/COD/Halo/Battlefield level in terms of popularity. At this early stage, where $500 for an Xbox One, or $450 for PS4 is no joke, will MKX make me want to splash the cash and jump over to a next-gen console?