Because most people listen to more different songs than they have (or need) different apps. If I have one to do app and am happy with it, there is strong disincentive to buy/use a different one.

Whereas if I have one hit song, chances are I'll buy another one, there really isn't any disincentive.

Or put another way: count the number of apps (you use) vs. the number of songs you listen to.