Cheap large portions of food, and sedentary lifestyles.
Fast food is cheap, and you can get lots of high-fat, high-carb, low-anything good for you food for very little.
Like Soda. Oh God so much soda. And "French Fries".
Most cities aren't well designed for active modes of transportation like bicycling, walking, or running. We also have a general consensus, in most towns, that mass transit is for "poor people". We sit on our ass in the car on the way to work, we sit on our ass in front of a screen at work, then we sit on our ass in front of the TV. Sleep, repeat. All the while taking in about 3,000-4,000 calories per day.
Also, even eating at home, there's this "Food Pyramid" thing that everyone was taught through grade-school for about, well, ever, sponsored by the United States Department of Agriculture that strangely exalted grains. As a result; people take in waaaaaaay more carbs than they should ("but it's not fat!", retards), but think it is "healthy".
So, commercial food, poverty, sedentary lifestyle, and poor nutrition education.