In general the reasons the USA is so obese is because we are a very large continent sized country that is not very pedestrian friendly.
People don't get to walk around as much and aren't burning calories.
Plus the kind of work the majority of the country does, For most people in this country the most manual labor they will ever do in life is taking out the trash at Mcdonalds. We used to be very industrial, and craftsman based with employment. For example i have one Grandpa who worked landscaping part time, and in a factory full time. My other Grandpa was a blacksmith. Both muscular due to their work. not many americans break a sweat at work these days.
Has nothing to do with diet though. We would still be fat if everyone was eating health food. If you intake more calories than you burn you will gain weight. Sitting around all day eating kale and not moving will still have you looking like Jabba,