Both Joel Spolsky and Jeff Atwood made several articles or blog posts explaining this. Obviously, they've got the licenses cheap; they're using the technology (WISC) they know best; and WS2008 is far, far better than any other previous MS server OS, due precisely to huge pressure from the linux challenger :)

Note : I'm a FSF member and free software fanatic, but I won't pretend that something that obviously works well, doesn't.