- Different pricing. Apps can be anywhere from $1 to $500, while songs typically hover around $0.99. The wide variance in app pricing makes a subscription model challenging to satisfy all users and developers.
- Different markets. When you make an app, you're tying it to be used on a specific platform. It is additional work to make it cross-platform. That alone is limiting for an apps' market, and in the case of iOS, the platform's owner can impose additional limits. Music is open-ish in the sense that you can make music once and release it in a way that it's playable, essentially, everywhere.
- Different revenue streams. Music is a digital good like apps, but it's also a physical good (CDs, records) and most importantly it's a service (live performances). It's possible to have additional revenue streams with apps, but it's not as common as it is with music. If Apple spontaneously created an app subscription model and your app was iOS only, you would suddenly be very locked into that business model, whereas with music you will always have additional options.
You're missing the biggest difference: longevity. An album can sell for 50+ years whereas an app... well, let's just say it's one step above "ephemeral".
All the more reason app developers shouldn't/wouldn't go for such a deal.
This is slightly off topic, but it seems that iTunes song prices have silently increased to $1.29, which is a huge increase. There are still some $0.99 songs out there, but most of the tracks I buy are $1.29.
Seriously though, here in Australia the regular price for a track is $2.19 -- which is absolutely ludicrous to be honest. As of today, that's the equivalent of $1.70USD
- Different pricing. Apps can be anywhere from $1 to $500, while songs typically hover around $0.99. The wide variance in app pricing makes a subscription model challenging to satisfy all users and developers.
- Different markets. When you make an app, you're tying it to be used on a specific platform. It is additional work to make it cross-platform. That alone is limiting for an apps' market, and in the case of iOS, the platform's owner can impose additional limits. Music is open-ish in the sense that you can make music once and release it in a way that it's playable, essentially, everywhere.
- Different revenue streams. Music is a digital good like apps, but it's also a physical good (CDs, records) and most importantly it's a service (live performances). It's possible to have additional revenue streams with apps, but it's not as common as it is with music. If Apple spontaneously created an app subscription model and your app was iOS only, you would suddenly be very locked into that business model, whereas with music you will always have additional options.