One line isn't even enough to tell the computer that something is attached; it needs a minimum of two. Considering that USB is either 4-pin or 6-pin, this leaves basic USB far short of what is needed for some functions. This is why other connectors are coming into style.

I'm fully aware of PCs that have 4, 6 and even as many as 8 USB connectors on the front and back of the case and I'm also fully aware that USB is capable of accommodating as many as 256 simultaniously-attached peripherals. However, the more items that are connected affect the performance of that bus AND affect the speed of communications to any one single device on that bus even when no other device is actively running. Simply the fact that they are there means the bus polls them looking for a change of state. While faster than the older Serial busses, USB is still a Serial bus.

Secondly, not all devices that something like the iPhone or iPad will plug into will be USB-centric. Most monitors and displays use some form of digital video--not USB--and send different signals on a minimum of three channels, not counting audio and sometimes separate gain and intensity channels. My iPhone 4 uses that 30-pin connector to feed sVGA, Digital Video AND HDMI to whichever system that display or projector requires to operate. What with, finally, the larger sVGA and Digital Video connections becoming obsolete, that is two different protocols and pin leads that are no longer needed. As such, the iPhone's own connector no longer needs to support them--allowing a more compact connector to replace it. However, HDMI still uses more than 4 leads and the iPhone's 9-pin connector provides what is needed for connection to multiple different devices--including USB.