Well full frame would be 12:9 and widescreen is 16:9 so they opt for a medium which means that people who have 4:3 tv's don't have to have huge borders top and bottom nor do people with 16:9 tv's have to endure large borders at the sides, just small borders for everyone.

As I said though with digital broadcasts they can transmit everything at 16:9 and then let the digital decoder have a setup to alter it for your tv.