Because it does what it's supposed to. I use W2k at home and see no reason to upgrade. I haven't encountered anything I want to run that won't. For the fires couple of years there were some hardware devices that didn't have drivers, but that went away.
Anyone who used windows prior to 2000 has reason to think it sucks.
The Mac platform has nothing even remotely suitable for our operational needs, so we're on Win2k Pro (although my office PC, part of the network, is WinXP Pro). That being said, I've never had the desire or need to own a Mac, and have tried to help enough Mac owners with their "simple" machines that I can't get excited about them at all.
Lots of flash and dazzle in MacmoonieWorld, but I'm trying to run a business.