I was considering this as I posted. The difference, I believe, is that apps have users and music tracks do not.

The best song I've ever downloaded has exactly the same functional requirements as the worst song. The interface is consistent. There are not versioning issues. The purchase, delivery, and use experience is consistent. Variation is purely due to content.

Furthermore, I can accurately make predictions about the quality of music based on previous experience. If I like Rock-a-billy, my expectations when downloading a new artist within the genre are likely to be met. This isn't true if I like first person shooters.

To put it another way, a music store contains a much lower percentage of poor value propositions. A song may suck, but it won't be useless, not as advertised, or a scam.

[edit] Pandora illustrates the phenomenological difference between songs and apps. The idea of an app which streams apps to users is incoherent.

Is a web browser an app which streams apps?

A decade ago, Apple fans said it didn't matter if the Mac had fewer apps than Windows, because the quality was higher. How times change.

As an Apple fan I stand by it not mattering if there are fewer apps. If the iOS App Store only had crappy apps in it, having 700,000 would be no consolation.