That's not really a fair comparison because your average Linux user and your average Windows users probably have very different skill sets when it comes to computers.
Your average Linux user probably installed it themselves and therefore admin their own PC. This makes them much more likely to have upgraded to a kernel >2.6. Your average Windows user got it pre-installed when they purchased their laptop/desktop and has absolutely no idea how to upgrade it. They'll stick with whatever it had when it first arrived and only upgrade when they get new hardware with a new version pre-installed.
The large Windows 7 install base also has to take into account the number of business users which are still buying brand new hardware (which probably comes with Win10) but then installing Windows 7 on it from some kind of image. Large companies take a very long time to upgrade to the latest version of even simple software, never mind an entire OS upgrade with all the regression testing that involves. My last company had over 60,000 employees worldwide and was just rolling out a huge Windows 7 upgrade when Windows 8.1 had already been released!