$500 to $1500 for a new computer is a lot of money for most of the world and even lots of folks in the more affluent contries. And as cdogg said, if it does what you want, why upgrade?
Though few still run pure DOS and Win 3.1 and Windows for WOrkgroups, and hopefully not many of fickle Windows 95, there's many Windows 98, 98se and Me that work just fine, thank you! And that's a big problem for Microsoft since they don't want to support older Windows systems' security vulnerabilities. Sure everyone with XP can get SP2 for the latest protection, but that still leaves all those older systems out there with either no more fixes or users who don't know to get the patches.
Personally, I've been itching for a laptop for the last 10 to 15 years, but since I often build and upgrade my system piece by piece, a laptop is a unit you've got buy all at once, not piecemeal.