It's because most obese folks driving in the US can only use an automatic. It's hard to shift holding a Big Mac and soda whilst talking on a cellphone which is held between their ear and shoulder.
The US has always been predominately automatic whereas in Canada and Europe prefer manual transmissions. Owner of both a SRT6 and Crossfire (manual). I have to say, I can roast my tires and shift far faster than the clunky automatic used by AMG.