The difference is that it is a lot easier for a developer to offer his app to app stores beyond just Google Play, than it is for a developer to port his Windows app to OS X or Linux.
I'm assuming that Google doesn't disallow distributing their apps via alternate app stores if they want them on Google Play. Someone correct me if that's wrong.
So the recipe for a successful alternate app store would seem to be enticing the developers of a lot of major apps on Google Play to offer them through your app store as well. The only way to do that is by promising them more money - that is, pay the developers of popular apps like Facebook and Angry Birds to offer them via your app store also. That would be pretty expensive, but might be the only way for someone to be successful with their own non-Googly version of Android.
Biting the hand that feeds IT
