Because for 98% of genres, it's completely worthless as a gaming device or anything other than a quaint gimmick that is a waste of development resources.
If you're into mini-game collections/fitness games, I guess it's something... as to the voice thing, I could have just as easily got something like that done with a simple voice box/headset instead of wasting valuable resources stuffing Kinect 2.0 into the next Xbox when we could instead get something that is actually worthwhile to games, like more hardware power (if Kinect 2.0 is indeed mandatory).
Putting aside the input lag and unreliability of gestural commands, the simple fact is that without tactile feedback and the full range of controller benefits, the input device is largely worthless for games.