Probably due in part to the 'Super Size' offer at most fast food places. As well, activity levels are dropping in relation to the number of game consoles and other 'sit-on-your-ass-all-day' toys.
Diet, however, is the biggest culprit. Europe and Asia have a much lower obesity rate because their diet consists mostly of whole grains and fish, as do all Mediterranean cultures. Cutting back on processed food and cutting pop out of your diet will likely increase your health ten-fold.