A traditional diet of very fattening foods, a culture of excuses and just general laziness. They all lead to the outcome. The very best solution to obesity is to never become obese. Bad genes? Great, we'll all cry with you for a few minutes, but you CAN change your metabolism and you CAN beat genetics.
It's strange that obesity is a lot more problematic amongst the poorer half of America. Seems pretty contradictory, doesn't it? Historically, its the poor people of the world that are starving to death, yet in America many very poor people are killing themselves with obesity.
McDonalds/fast food, doughnuts and other such things aren't helping matters.