Some possible reasons for not utilizing web browser (http) protocols and the EDGE network:
-The user authentication for iTunes is not based in https; Python is the underpinning and the primary programming language for iTunes app, and the iTunes store’s authentication mechanism is not http: compliant. Whenever you click on links from http web browsers on a computer, they are scripts that open iTunes app, so the same would apply for mobile implementations of iTunes;
-Another possible reason is that the download packet size limit for Safari on iPhone is capped at 8MB; most files and packets in iTunes exceed that maximum;
-GPRS and EDGE are too prone to errors and packet loss for high-bandwidth files, especially when the files are only downloaded a single time in a given transaction. The iTunes servers are optimized for robust delivery of files/media so as to make the traffic as efficient and cost-efficient as possible.
-Lastly the DRM mechanism needs to be delivered/encrypted over a secure protocol, and http is to susceptible to hacking.
These are my best guesses, but a more programmer-type of flutherer may correct me on some of these points (feel free btw).