easy to explain
In most cases, Windows is a defaut. You want a computer, you do nothing special in the process, you get a Windows PC.
Every other result, Linux, Mac, Commodore 64, or something else requires a proactive choice. And its very much human nature to defend one's choices. Particularly when cash or some other serious commitment is involved.
Its not just OS.. you get similar battles over Emacs vs vi, Chevy vs. Ford, Canon vs, Nikon, Canon vs. Sony vs. Panasonic, PS3 vs X-Box 360, Coke vs, Pepsi, etc. That's human nature, but only when it's a choice.