Windows Phone's market share in the U.S. is at 3.4% (and apparently isn't growing)[0]. Supporting your apps on another platform is expensive, so I imagine that Google doesn't support it because the market and users aren't there (relatively speaking). And, of course, Window's Phone users can still visit the mobile YouTube site, which (while I haven't tested it) I imagine supports Windows Phone fine.

If the market share increased significantly, I'd be surprised if they chose to continue not to support it. But for the time being, their decision makes sense to me.

[0]: http://techcrunch.com/2014/07/03/windows-phones-market-share...