The Atlantic says it's because we don't walk enough. Which is true, but 1994 wasn't that long ago and I really don't think people were walking any more then than they are now. I jog and when I check my RunKeeper stats it reports that I've burned up maybe 500 calories after 45 minutes. Fast, steady walking may burn 300 calories in 45 minutes. So a lot of walking doesn't burn that many calories.
I am more inclined to believe it's the sheer volume of crap we are eating. Portion sizes in restaurants are enormous compared with 20 years ago, and so much of it is laden with HFC's. I don't think corn syrup is necessarily bad, but they are mixing it with everything now in order to make food taste "good."
What do we feed cattle in order to fatten them up?