Well, that is to be expected since OS X updates are all free and installed like any normal application update whereas upgrading to a new version of Windows is expensive. Unless you are using a very old machine that doesn't have the hardware support for the latest version of OS X there is no reason not to upgrade whereas for Windows, most people would rather not spend around $100 to upgrade from Windows 7 since there is no massive benefit and if they have an older machine with Vista or XP it's not going to be worth spending that amount of money on.
I imagine this will change when Windows 10 is available as a free upgrade to Windows 7 and 8.1.