It is hard, really, really hard, to convince game developers and publishers to make games that require an optional accessory. It's been tried again and again and it is never more than a mild success and usually an abject failure. They want to target the base platform, which will always have a dramatically larger install base.
Thr original Kinect was just such an optional accessory, and yeah, didn't have much in the way of third party support.
The XB1 could have, and would have, encouraged more devs to use and experiment with the Kinect because devs could be assured that every single user would have one hooked up.
Except that Microsoft chickened out instead of sticking to their guns. As usual.