Everyone knows Windows these days so that's all they use. I think if Unix is to take over it'll have to start in education. That way the kids learn it as a "first" OS rather than Windows.
The issues are the actual switch to Unix and software. It would take time to cycle all the Win computers through to Unix. Also, as with Mac's, there is hardly any software for it. There is such a large software base for Windows that it isn't possible to keep all the apps when switching to Unix. I speak from experience as I work at a school district. That's my 5 cents anyway. :p