Why do we have a duopoly? You can draw a direct logical line to the complexity of shipping apps through an app store. Right now, an app, say, Spotify, has to create two versions of their app (iOS and Android), and package them for the Apple App Store and Google Play Store.

(Buuut that's not even the whole truth. Mobile OS updates frequently break backward compatibility, so iOS and Android apps need to gracefully handle backward and forward compatibility, often special-casing different OS versions or screen resolutions).

Anyway, if, say, Amazon resuscitated their Fire Phone, which would use the Amazon App Store, then Spotify would have to package their app for the Amazon store... which costs developer (or at least a release coordinator) time. That's a barrier to entry that doesn't exist in the PC ecosystem.