Distributions freeze the set of available app versions to a specific version of the base system. You cant get a new app without upgrading everything else too, including all other apps. I cant simply get a new Emacs on my Ubuntu, because to do that, Ubuntu also wants to remove my Gnome2 and replace it with Unity.

Windows decouples the base from the apps. When you want to update an app, you just do it, everything else remains untouched. If I want to get a new Emacs on XP, it wont force me to simultaneously upgrade to 7.

Windows would have the same problems if they bundled a new set of base libs and 20000 apps that only work with that specific set of libs every few months but they dont. They invest a great effort into making the base system slow and stable and support it for a decade or more, and it shows.