Because the mass change from xp to windows 7 has finally waken Microsoft up to the fact that no one (ok, the vast majority) likes windows 8 and they'd do better by keeping up the support for 7.
While the API may have the same name, it doesn't mean the low level code is the same on Windows 7 as it is on Windows 8. Is it cost effective to create 11.1/2 for Win7 or just get on with redesigning it altogether for DX12 for both OSs?