Well, it's just a .1 difference anyways. Were not talking about a FULL version change here like 10.0 - 11.0. I don't see any reason why 11.1 couldn't be implemented in Windows 7.
Cost-benefit. 11.1's selling feature is native (DirectX) stereoscopic support. As DX9 is still far outnumbering DX11 applications, the cost to bring the new features to an OS that lacks the underlying support specifically written for the .1 updates for 0 real world applications was not deemed appropriate. Same will be true in 6 months when we are still sitting at 0 real world applications requiring 11.1. 11.1 was needed for Win 8's interface.