The reason is that the iPhone’s HotSpot function works so well. Well enough, in fact, to make the idea of a dedicated port redundant. Better leave the remaining side estate for a more deserving port.
I have been using the HotSpot function more and more in conjunction with my MacBook Air. By far the most sensible way of doing it is by direct cable connection from the iPhone to the computer’s USB port. Not only do you get instant internet on the laptop (after activating HotSpot on the phone, of course), you have constant charging of the phone instead of a continuous battery drain if you use wifi. It’s also more secure than wifi when sitting in Starbucks.
A word of warning: Some carriers expect you to register for use of the HotSpot feature (also known as tethering) and many charge extra. Either way, it’s a cheaper option than maintaining yet another SIM card for the computer.