It's simple: As a standard-issue, always-on-and-enabled FEATURE of the Xbox One, Kinect was something that had a hugely better chance (not a guarantee, but a good chance) of being developed for in a meaningful way. In a way that is more than a really neat trick in a controlled environment — like, say, the way the Kinect is currently deployed — but that you won't use in real life because it's not any better than just using the damn controller.
But as an optional feature, there's far less incentive for developers to take advantage of everything the new Kinect can do. It's actually a really impressive device, and maybe some of that still comes through. There's a good chance it does, actually. It's just much lower than it would have been otherwise. Instead of mind-bending progress, we'll get slightly more evolved Kinectimals.
Now as far as we know, you still have to actually buy the Kinect. You pay for it and own it and can plug it in at any time and activate it and enjoy it. But so many people won't, wilfully, or simply don't give a shit enough to plug it in, because beyond voice controls, there really aren't many compelling reasons to really use the thing aside from that one time a year your friends come over and dance like idiots and spill wine on your rug. And if the £100 difference between the PS4 and Xbox One begins to make a difference, how long will it take for Microsoft to begin shipping units with no Kinect at all? It can claim that won't happen, but at this point, what credibility does it have that it won't, under sufficient pressure, offer a first shipment of Xbox One consoles crammed full of My Little Pony action figures some Microsoft exec read that people like on 4chan or something? None, really.
And you know what, yes, as the layers of the NSA/GCHQ shitshow have been peeled back, the possibility of the NSA compelling Microsoft to give it access to either metadata or outright reams full of data seems more and more viable. That's chilling for a device capable of the fidelity of imaging and sound capturing as the Kinect 2. But shutting down Kinect because of it is anti-technology, anti-future. It supposes a technological Tower of Babel, wherein there are some things we just aren't advanced enough to use. We're not tall enough to ride always-on technology.
Maybe the fears that are being assuaged here outweigh the development boon of mandatory always on. That's perfectly possible. But what's definite is that an optional Kinect is an ignored Kinect, a £100 tax on a device that's doomed to never live up to its full potential. Most of all, it's just a damn shame.