It can be much easier to separate people from their money using the App Store model.

As soon as you throw up a long-ass payment screen, where I have to fill in name, billing address, CC info, etc, a lot of people will bail, both out of distrust for you and sheer inconvenience (doubly true for platforms with meh text input like phones and tablets).

Compare with integrating with a payment service that is almost guaranteed to already be set up correctly and is a single password away from transaction.