One must also take into consideration the fact that most PC users are not very computer literate, and some form so tight a bond with their operating system that they will fight logic to use it. I can't count how many times I have argued the benefits of Macs vs. x86 boxes, proved the other person wrong, then watched as they became red-faced and hissie-fitty. Some people just don't want to or simply can't learn to use a different OS.
Then there's the whole compatability issue. Right now, I am unaware of any linux distro that supports windows apps out of the box. One is forced to install and configure things like wine and vmware on their own - something daunting if not impossible for 'the average user'.
The bright side to 'everyone else' using windows? As unjustified as it is, it makes some of us feel big