I mean... WINE does the same on windows, but microsoft refuses to release their API docs for all internal APIs. They release WSL by relying on Linux's open-ness, while refusing the same for themselves.
A big one of those reasons was Docker. Docker was still fairly niche when WSL was released in 2016, but demand for it grew rapidly, and I don't think there was any realistic way they could have made it work on the NT kernel.
