Some home grown programs and other older programs still only work on XP. Also because of the fact that it is a capable OS at web browsing and at actually operating a computer, some people just do not see the need to change.
Ideally everyone would be running on Windows 7 and up now, but not everyone wants to stay up to date.
It's a bit of a cope out to be honest,for browsing etc you can do that with any Linux and more so costs nothing and better security etc...
As to old software programs,in most cases there are newer versions(I know for a fact since I upgraded a few over the years).
What it boils down to really is cost for some companies and users, plus can't be bothered to change so that simple IMHO.