I don't know for sure, but size is definitely a factor. If you buy a new phone and it fails to fit in a slot, holder, or dock in your car, then that's not doing anyone favors.
Same thing with any other accessories too. If it won't connect to your home stereo, or a dock, then in my eyes, Apple isn't doing me any favors.
I guess this will all change when we finally get to a point where iphones and tablets don't have any external connections at all. Then, the "dock" would become nothing more than a flat place the wireless charging could occur, and the audio wouldn't be fed through a DAC in the phone, it'd be a audio bitstream and processed outside the iphone, with whatever flavor DAC a customer wants. Seems WiFi and AirPlay2 can accomplish this pretty well.