From what I've read at Doom9 and on previous threads here, the basic deal is:
1. Prior to Windows 8.1 (or maybe Windows 8.0, that was unclear), the implementation of 3D support under Windows varied depending on the GPU manufacturer.
2. Since Windows 8.1 (or, again, maybe 8.0 did it), Microsoft implemented a "unified" system, but adoption of Windows 8 has been, obviously, less than enthusiastic.
3. Madshi was unwilling to spend a massive amount of time to implement GPU-driver-specific implementations, only to have them broken by a future driver update, and was unable to easily implement a single solution that would work across all machines, prior to this.
4. He was also less-than-enthusiastic about implementing something that would be limited to only Windows 8.
And, of course, 3D is a gimmick, so there's that.
SBS 3D works fine in MC on my Panasonic, by the way. That's the only time I've ever turned the feature on, but I did as a test, just a few weeks ago. Worked okay. Turned it back off and put the glasses away and they'll probably never be used again.