I'd add a few more things to this:
-THE PILL. For a lot of women, birth control pills cause weight gain.
-WORK/CAREERS. Look who now enjoys the "benefits" of an entire lifetime of being in the workforce, being on the run, coping with the stress of jobs, etc.
-SOCIAL MEDIA & TECHNOLOGY. In the past 20 years, in particular, everything has become so much more accessible and less labor-intensive. Combine that with society's seemingly being addicted to have our collective noses in a screen and asses on a chair/couch and the combination for calorie burning is shot.
I don't think it is ANY wonder where we are today. During trips to other countries, you can see stark differences and, overall, stark differences in the fitness level of people, how they prioritize a work/life balance, etc. This is not surprising in the least to me.