Like it or not, Microsoft wins a lot of points for supporting older versions for a decade plus. They might lose a few bucks on a new Windows license but make it back on reputation and other enterprise tools.
The world is changeing and the enterprises will have to adapt.
That means large numbers of organisations were still installing Windows XP as their standard desktop around 3 years ago, and possibly even more recently than that since they wouldn't necessarily have evaluated Windows 7 and set up to migrate to it from day 1.
Moreover, the days of upgrading hardware every 2-3 years are gone, at least in typical office environments that aren't really pushing the performance limits of a typical office PC any more.
In other words, for large organisations that keep relatively up-to-date, it's still entirely possible that they have a lot of WinXP machines still around today, even if their newer machines are on Windows 7 now.