I agree with some that this is a combination of power needs and licensing. That $300 fee for the Kinect for windows early dev program is a bargain if you break it down. You initially get the beta/dev unit but then you ALSO get the final unit later on. They're giving you two of them plus the SDK plus direct support from the people who make the damn thing. That makes the price tag pretty cheap.
Also as far as power goes, I'm betting it uses more than v1. The cameras are in HD now, but you also have other abilities the v1 didn't, like seeing in the dark, etc. There's more logic stuffed in there I'm sure, in order to make up for any limits on the Xbox One itself. Remember the Kinect 2 on the X1 has to do all that without getting in the way of the console itself that's playing a game and doing other tasks to, which again means it probably has extra bits in it to help process all that data. The windows version coming next year will probably not, since the developers can use more of their PCs CPU/RAM/GPU etc, they're not going to have to balance the Kinect + running some new AAA game at the same time. This probably means the Kinect 2 for windows units they ship in final form will be smaller even, use less power so that USB3 supports it, and so on.
The final part being costs and licensing or however you want to think of it. No way the new Kinect comes in at just $100 with $300 being for the X1 alone. Not when the v1 Kinect is sold for $150, so you know MS is taking a hit even at $500 for both. This means they want to make it up with the software/games not have a chunk of them being bought only for the Kinect and seeing the X1 tossed aside.