It looks like Sony paid for short term exclusivity
https://twitter.com/SusanCStieglitz/status/723202449960366082
It's pretty disappointing that Sony paid to get the mode first when we've supported the game since December. I probably won't be playing the game anymore by the time it comes out on Xbox though. I've already significantly reduced my playing time over the past couple weeks. It just doesn't have the same magic as it used to. I guess that just happens sometimes after playing a game 1000+ hours.