Why - because the vast majority of new systems are shipped with Windows on them, and MS indulge in some *aggressive* business tactics to try and ensure this remains so.
I'm almost switched to using it myself - but there are still some programs I am used to, which I will switch back into XP to run. It's not as easy as falling of a log to make the switch. Though it is simpler if one starts from scratch with Linux.
The new $100 childrens *laptop* will, IMHO change things dramatically in the years ahead. a starter run of many millions of laptops all running Linux is bound to have an impact eventually.