It would risk diverting attention from the Unix port to the Win32 one. Even if the Redis developers don't pay attention to this distraction, its mere existence fragments the codebase and creates two semi-compatible versions.
And if the Windows version sucks badly, any Windows user who tries to install it on Windows will end up blaming Redis and try something else. On Windows.
If, in the end, Redis' codebase becomes cluttered and performance and maintenance suffer, we all lose. </quote>