>chemicals and processed shit
High sugar and starchy foods are more accessible and taste better. Go to Wal-mart and that's the first and last thing you see. Even the flour that makes up the carbs we eat is overly processed. And most of all, the shit is everywhere. Sugars are addictive and become something most people can't break from.
>tradition of poor eating habits
It takes a lot of effort to break from the foods most of us in the US grew up on. Most people refuse to eat salad and encourage the mentality. Foods kids eat turns into "comfort" food as they grow up. Thus as most kids grew up on chips and sweets, they fall back on that during stressful times. Also these foods are staples in the American diet - such as fries, burgers, etc. Most people don't take the time to cook. People who don't cook often think it takes a lot of time to prepare food, and thus never develop the experience of fast cooking.
>food and habits create class distinction
It's true. Lower income families and people of certain states, cities or cultures associate what should be normal life habits as "bougie" upper class Hollywood stuff. They don't think that this type of thing is for them and are complacent with the life they live.
>transportation differences create less physical exercise
The biggest difference between US and a lot of other countries is the fact that we have such expansive lands that most people can't get around without a car or bus. There are urban cities that do require more walking, but again, we have cars.
I've visited other countries and much more walking is involved. These people don't even have to exercise to stay lean.
>increased acceptance of fatter bodies
Ex-fat myself. I'd be considered fat in some countries, but here, I'm slightly below that in this country. Advertisers capitalize in selling to the norm now, and thus more fat acceptance. In general, since it's much harder to get a whole country to change, might as well change the way they market.
>full time office jobs
Add it up and a full time job means taking away 10 hours of your day. That leaves only enough hours to get something to eat and having some personal time before you roll into bed. Most people do not want to use their precious time working out or sorting out their lives for something healthier.