Version-specific sites are a bad idea because the software will be deprecated eventually, so the community would die. Assuming you didn't mean a version-specific site...
One reason there is no Windows site is because if all the Windows questions and answers left, this site would lose much of its audience.
https://superuser.com/tags shows that Windows is the top 2 tags.
Windows (or version) tagged questions make up around 32% of questions on the site.
