There's a bit of a bet going on in Apple's mindset. For most people, USB drives are no longer a thing (Dropbox, mail-to-self, etc.), so why put ports in the front? If they're in the back you can set up your keyboard cable or w/e nicely and keep those out of the way.
_If_ your USB usage is not 'often plugging/unplugging stuff into the port', then having the ports in the back is better for usability and the like.
Of course this doesn't work if the bet they make is wrong (see MBP dongles)