I already said it in another thread where a PS4/Xbox One version was speculated, but: I don't think the lack of a Xbox One version has anything to do with hardware or software sales at all. Both the Kingdom Hearts Collection and the Final Fantasy X HD collection use Sony proprietary tools, and that's why they were never released in any non-Sony platforms.
If they could do it, we'd have seen at least one of the collections hitting X360., or at least having a PC port announced.
So yeah, even if Xbox One was outselling PS4 i really doubt this would be on it anyways. Which is a shame because this should hit as many platforms as possible, especially with how a mess it was originally to be able to play all the game in the series.