Simply because
It works well with the vast amount of PC hardware/devices of choice that I have.
I really make use of the fact that I can get reliable Microsoft tested and certified drivers for all my devices without spending a single minute.
My main workstation is a self built one - not only for the reason that it works out cheaper but also for the fact that I can choose what goes in it. And Vista x64 works perfectly fine on this quad core box.
I use Linux on my headless mac mini server because most development software that I use requires Linux/Windows/Solaris. I am also more comfortable with Linux and it just works.
Mac OS X is on both my and my wife's laptops (Macbook Pro and Macbook).
So in short OSX for everything desktop, Windows for Workstation and Linux for Server - right tool for the job!