Papa Johns stopped sponsoring the NFL. THEREFORe the average American could not get the healthy vegan pizza once provided to them by Daddy Johny. therefore, they would seek out vegetables, thus making them big and fat.
But cuisine in other countries is just as fatty, or worse such as in the UK, but they have less of an obesity problem. The difference I think has more to do with walking habits. In America, people seem to avoid walking as much as possible. They will drive over to the next parking lot if it saves them a minute of walking. There is significant infrastructure devoted to avoiding walking, like the placement of parking lots and availability of scooters. This does not seem to be the case in other countries, at least not to the same degree.
Combine better activity habits with a better diet, and you get results like you see in Japan or Sweden.
Culture plays a large part in the obesity of a country. As one time, Americans were not considered to be overweight. As women were entering the workforce, less and less time was spent at home. Now, two working parents were away from the home and there was less enjoyment of cooking dinner for the family. Plus, the children have several activities that may be taking place during dinnertime.
With many restaurants being built over time and with many of those restaurants being fast food so that the mom or dad can simply pick up dinner. However, many of those fast food restaurants only carried fried, fatty foods or foods that were high in sugar content. Eating these meals several times a week would lead to an overweight family.