Whether it's XP, 98, 95, NT, OS9, SCO, OS/360, etc. isn't really the issue. It's that customers need a reason to upgrade, and making life easier for a web developer is usually not that reason.
Or, for a significant number of businesses, support for some expensive custom hardware that came with a software package that runs on (for example) Windows XP, but doesn't work because of the device driver architecture changes on Windows 7. The odds of places like that upgrading just to get a new version of IE are approximately zero, and even less if part of that software solution actually uses IE specifically.