The Linux syscall ABI is pretty small and stable, and once you have that any userland program just works. Windows (NT) doesn't have a stable syscall ABI, instea

The Linux syscall ABI is pretty small and stable, and once you have that any userland program just works. Windows (NT) doesn't have a stable syscall ABI, instead having stable ABIs for libraries that wrap it in various ways (Win32, most prominently, but historically also a POSIX library). As such, WINE has to reimplement all those libraries, whereas Windows only has to implement something comparatively tiny. I expect someone can write a better summary of pros/cons of both approaches (and there are definitely both for both!), so I'll leave that to them!