Because Play Anywhere is awesome.
Sure, in concept except the store has many many reports of being well, horrible. Slow downloads, which for gears4 is 80gb, annoying interface, admin/login issues to get it to work etc.. THe only way I can get gears4 to play on xbone and win10 is to buy the digital version for xbone. I don't know why I can't buy a disc copy and then use that with an associated win10 account.