All of the above (Score 1)

But most of all because (IMHO) it's not as easy to use as either linux or OSX. I love Linux because I get all the control I want at 0% of the cost. I love OSX because it does everything windows is supposed to do but more intuitively, in my experience faster and with gratuitous use of transparency. I love the two together because they play nicely. Personally I don't see a niche for Windows in my future.