Apple is known to like to exercise control and to keep their ecosystem closed (recall that Jobs was initially AGAINST the AppStore concept entirely, which was invented by someone else and now makes a ton of money for Apple).
Open systems with little control lead to a bazar like the various Linux distros with inconsistent-looking and buggy apps mixed with great apps - anything goes.
Imagine you are Apple and you want your AppStore to convey a professional and safe look and feel, you probably don't want 60,000 converter or image viewer app but a variety of tools, maybe 5-10 per category max, or it will be hard for users to navigate and tiring to explore.
Jobs was initially against allowing third-party native apps on the iPhone, but not App Store as a distribution mechanism. Once he relented on inviting ISVs to the profit party, the iTunes Store model was never in question.