If millions more people are obese than were obese 50 years ago, clearly something has changed systemically that has made people more sedentary and eating more. People 50 years ago were not paying more conscious attention to their health than people today, but the background environment of available food and sedentary jobs/entertainment were different.
The personal responsibility model of obesity works for individuals (including myself), but falls flat when discussing how to lower the weight of millions.
“Freedom works for me but I want to also control millions of other people, for the good of me and their own good since they don’t know any better”
What changed 50 years ago is the US government decided saturated fats were bad and complex carbohydrates were good, and began setting policy to rebuild the food supply and culture around that worldview. We're now living in the result of that population-wide experiment.