It's probably several things: - USB plugs are not double sided, leading to confusion when plugging them in, and often resulting in material fatigue because peo

It's probably several things: - USB plugs are not double sided, leading to confusion when plugging them in, and often resulting in material fatigue because people force to plug them in the wrong way (iPhones are plugged in much more often than - say - a USB mouse that stays connected most of the time). - USB plugs are either a host or slave, not both. On an iPhone, the dock connector needs be both: slave (when connected to a PC) or host (when a USB camera is connected through the camera connection kit). - expected functionality through form factor: this plug will provide audio in + out, video in + out, playback controls etc. Normal USB plugs don't guarantee that functionality. Some cars have a generic USB plug to connect to generic USB storage devices to play MP3s from - now those owners might expect the next iPhone to be compatible with them - but these cars with the generic USB plugs might not understand audio over USB. Other cars in the future might contain your proposed next generation iPhone USB plug, leading people to expect that they can connect any USB device and transfer audio over it.