It has USB, power out, serial control, audio, video out, and remove features. Probably some others I can't think of off the top of my head. This means it's doing a lot more than just USB.
There are a lot of products like FM tuners and other aftermarket offerings that are only possible via the dock connector. A USB connection would require the dongles to be much more complicated and work a lot closer to the hardware.
PS If you made all the Audio digital over USB you'd need more complex hardware. You want to have the cheapest headsets and output cables to get the audio and video out of the machine. That means old fashioned analog audio/video. It's all in one connector so you can "dock" it with simply one connection.
There are a lot of products like FM tuners and other aftermarket offerings that are only possible via the dock connector. A USB connection would require the dongles to be much more complicated and work a lot closer to the hardware.
PS If you made all the Audio digital over USB you'd need more complex hardware. You want to have the cheapest headsets and output cables to get the audio and video out of the machine. That means old fashioned analog audio/video. It's all in one connector so you can "dock" it with simply one connection.