Craig Hockenberry, in a post on A List Apart, on web apps vs. iOS apps:
iTunes offers you a simple way to charge users for content. It can be a one-time payment via app purchase, or a recurring payment (such as a subscription) with in-app purchases. In either case, a customer only has to tap on a buy button and enter their password. Apple handles all the payment processing and accounting. You just wait for bank deposits from around the world at the end of each month.
My guess is this has a lot to do with it. As Hockenberry also points out, it's easy to pay for things on the App Store. There's no typing of credit card numbers and billing zip codes. Just enter your iTunes password, and you're done. Easy as pie.