[larrik]

Microsoft Windows's primary competitor is... Windows. Not OS X. It isn't easy to convince regular users to use the new Windows, or to do an in-place upgrade (which is hundreds of dollars, unlike OS X, and also a terrible idea usually, also unlike OS X).

Apple is simply pushier, dropping support for old versions almost immediately. They also release more often, and I would say that over the same span of time their change drastic-ness is similar (intel, cocoa, app store, etc). I wouldn't be surprised if 10.9 vs 10.5 is as drastic as Windows 8 to Vista, for instance.