This is just my take. The average intake of the American consumer has increased steadily, I believe. However, weight gain happens at the margins. If you eat 2200 calories a day and burn 2000, you are gaining weight twice as fast as someone who averages 2100 with the same burn rate, even though your intake is less than 5% higher.
As the average intake passes the burn rate, a little exercise helps. However, with a small percentage-wise increase in your intake, you can double or triple the amount of exercise required to get back to equilibrium. My guess is that in the 60s and 70s, adding exercise worked for many people. Now, the necessary exercise is much, much higher and caloric restriction is necessary in a way it wasn't for the average person 30 years ago.