I believe the answer to that is multilayered, but essentially, it is the fault of pre-prepared foods.
The adulteration of foods, the added chemicals, the highly refined calories, the preservatives.
Remember the main point of preservatives is to prevent fats from oxidizing. They don't quit just because they get eaten. Plastics, the toxins inside them affect our endocrine systems, stimulate estrogen etc.. All our food is stored mostly in plastics, served in plastic and we wear plastic..None of this existed in the 50s or 60s, and was just starting in the 70s.
Combined with a far more sedentary life. The majority of jobs shifted from manual labor to desk jobs. We all drive or get driven everywhere we go. That was not the lifestyle in the bygone eras. They actually did have to walk miles to school. Walking miles everyday and working on your feet burns a lot of calories, and keeps your muscles stronger than sitting.
I could go on...