Generally speaking, most companies are making decisions about their IT infrastructure with just worker "productivity" in mind. There's a whole host of other thoughts that go into the decision, including the raw inertia that Windows has maintained since the late-80s. Add to this IT department fretting about interoperability, training issues, and just an outright prejudice against other platforms, plus the lack of some software on the Mac, and we end up in a situation wherein while the Mac may be superior for productivity, it's still not the choice that gets picked.
Of course, productivity is a fairly subjective term. I'm much more productive on a Mac because I find Windows difficult, strange, and sometimes just buggy while I can use the Mac with aplomb. However, there are others who find the GUI on either Windows or OSX just completely unbearable and still others who love Windows '98 and will never upgrade (not to mention those OS9 hold-outs).
*I'm* productive on OSX and I find Windows Vista too horrible to contemplate, much less actually use, but I'm not blind to the fact that some people really do like Windows.
Also, that 95% of people like "X" is no measure of quality, its sometimes simple ubiquity.