Actually, from what I've been told by people who know more about this sort of stuff than I do, the Xbone isn't BC because the architecture is too different. Microsoft actually tried BC, with limited success, apparently. Or if they were successful, they've kept it to themselves.
The 360 had some serious emulation problems as well (some, less popular games can't be played on it), and even popular titles have issues. The early PS3's weren't emulating, they actually had PS2 parts in them. Though apparently the Slim, can brute force emulate PS2 games, with considerable modding (and from what little I read, it's not an exact science).
What is interesting though probably only I care is that the Xbone and PS4 are both powerful enough to brute force emulate Xbox and PS2 games.
But, there's really no reason why they would make that a feature, since they want to sell those games to you again.