The reason Twisted is available on Win32 is because (a) there are developers willing to test and maintain the OS-specific support for Win32, and (b) some people

The reason Twisted is available on Win32 is because (a) there are developers willing to test and maintain the OS-specific support for Win32, and (b) some people deploy apps which use Twisted as their general networking stack. (Deluge is one popular example of a Twisted-powered GUI app.) I bet the former is the big reason; I don't think people write full user apps with GUIs in Node very often.