probably because their focus is on data centers that mostly run linux
Since they cancelled the work on zluda and absolutely fail to do anything about other options, I really believe there's some "don't do it or you'll get sued to hell and back" agreement. They can't be so dumb they just miss it by accident.
It almost make me wonder. Is there a shady trade somewhere to ask amd never release sdk for Windows to hike the price of nvidia card higher? Why they keep developing these without release it at all?