Alright, I'll bite.
Backward compatibility, is the answer. Hundreds of millions of pieces of hardware that work with Windows but don't work with linux or OSX or *BSD. Billions of pieces of software that work on Windows but don't work on linux or OSX or *BSD.
Literally trillions of man-years of effort to make all that hardware and software compatible.
I don't think you realize what a staggering achievement backward compatibility in Windows actually is. Nobody else does it. The linux kernel introduces breaking changes with every major release. Apple are quite happy to fully reboot their whole OS.
Backward compatibility is what every industry that actually makes real things needs. Billions of production lines, control instruments, diagnostic instruments, even basic warehouse handhelds rely on it absolutely.
That is your answer.
I fed the troll. I apologize, everyone else.