Because I already have an operating system from Microsoft that I am not happy with.
You are making a huge mistake then. I have been running W7 since May with absolutely no issues.
Now have W7 Ultimate on all of our work machines (we are MSDN members and got early release last month). it uses less than 50% of the resources of Vista, boots within seconds instead of minutes, and uses about 50% of the HD space as Vista.
It is fast, user friendly, have yet to find one compatibility issue.
To be honest I am surprised it is a MS product, it is that good.
Have already pre-purchased home premium for all my machines at the house. They will all be loaded tonight.