You are kidding, right?
Forget macs models that really need this, look at the market logically.
Almost every external device has a USB. Very few of them have a TB. The current iMacs have two TB ports. How many people do you think have that many TB devices? How many have more TB devices than USB ones so are short on ports?
Ok, not the latter is not the best comparison since USBs can use a hub and TB uses a daisy-chain but you see what I mean and the first example stiull works. Having one of the TB ports be a USB is mush more preferred by most people. Not to mention that there are advantages to having a second USB bus.