Modern web programming is almost always deployed on Unix-like operating systems / Linux. The biggest reason to allow running Hare on Windows/macOS would just be for developers who don't use Linux.
Keeping the scope niche/small seems like a feature to me, not a bug. Let non-FOSS OS users find something else (?)
I am personally no FOSS zealot but I can respect a project that has a firm idea of its scope and targeted user base and wishes to stay limited to that
Not everything needs to be huge or growth hacked to death