I honestly wouldn't be surprised if either part of the issue is Microsoft's minimum physical print limit (tier rewards include physical copies of the game). After Final Fantasy Type-0 bombed physically, a lot of smaller and/or niche games are 100% digital on the Xbox platform.
I also wouldn't be surprised if Xbox throws some money at Platinum to also get the game on launch day, but to also have it on Game Pass (which I could also see causing drama for those that backed it on Kickstarter).
I'm also annoyed there is no stretch goal already for the platform, but I think they realize at this point they will need to release it on Xbox too, its just a matter of when they'll announce and release it.