The reason why they release patches so often, iTunes is the app where all their management of iPhone and iPad occurs, which means they often need to update it to add support for new iPhone/iPad/iPod touch models, and to support new features in those products. The fact that it is also where they handle a lot of their DRM (which they may have contractual obligations to update in some instances) probably is part of the reason as well.
The fundamental issue with delta patches is that they are more complex, and have higher support costs for the vendor. A full updater can run against any version of the app that is installed, a delta patcher most be made against specific version. A full updater will work regardless of whether the app has been modified (user hacking the binary to do something, tweaking the artwork, etc), a delta updater will fail against a modified app.
Given the extra work involved in preparing a delta patch, and the increased chance that it won't work, they tend to only be used in very specific cases. Apple tends to create delta updates for software updates, but they only create a delta against the most recent released version of the OS, rather than making dozens of versions that work with every release they make the one that will get most people, and then for everyone else they do a combo update that is a full updater that also incorporates every previous software update as well.
For smaller things like iTunes, they don't tend to bother with delta updates at all due to the increased workload, support, the increased number of versions that exist, and the fact that iTunes is fairly small in the scheme of things (it is smaller than an OS update, or an iPhone/iPad firmware).