having to pop open the backplate and fumbling with that microsd bay door everytime you want to exchange data would stress the hardware big time. microsd cards are usually not rated for flash leveling but times of ejection before they will break.
also, most people with better cameras already spent a nice sum on expensive sd or cf cards. having to buy even more, slower flash cards instead of just plugging a cable into both devices will be a biq inconvenience.

of course i can buy a bluetooth keyboard, mouse, gamepad and whatnot for at least double the price of a similar usb variant.

of course i can use the cinch video out cable with 480i instead of the possible 1080p of an usb-to-dvi adapter.

of course i could buy a 20-pack of microsd cards instead of bringing my portable hard drive along.

but, you know what? i could also wait and buy A DIFFERENT DEVICE that supports usb host mode.