I'm not familiar with MacOS 9 interfaces, but there are so many implementations of Unix out there (including WSL, Cygwin, Linux, BSD, and another dozen independent implementations) because the Unix interface was, on purpose, very simple. That is, in part, what allows Fuchsia and Darwin to be feasible.
Windows, on the other hand, is not. It has thousands of system calls which aren't really layered in any way. Wine and ReactOS have been at it for over a decade each and they still have "not yet implemented" system calls.
Microsoft has the "reference implementation" to compare with, but the amount of work is enormous even for Microsoft.
I'm not familiar with MacOS 9 interfaces, but there are so many implementations of Unix out there (including WSL, Cygwin, Linux, BSD, and another dozen independent implementations) because the Unix interface was, on purpose, very simple. That is, in part, what allows Fuchsia and Darwin to be feasible.
Windows, on the other hand, is not. It has thousands of system calls which aren't really layered in any way. Wine and ReactOS have been at it for over a decade each and they still have "not yet implemented" system calls.
Microsoft has the "reference implementation" to compare with, but the amount of work is enormous even for Microsoft.