It's probably multifactoral. For example, here's a recent provocative paper from a highly respected researcher (John Speakman) arguing metabolic rates have actually slowed since then: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC10445668/

But two decades of "fat doesn't make you fat" probably didn't help, and neither did the continued trend of increasing empty calories from cheap vegetable oils, especially soybean oil: https://thedietwars.com/why-are-americans-getting-fatter-a-f...

https://www.ers.usda.gov/webdocs/charts/59529/indicators_goa...

>Added fats and oils provide more calories per day for the average American than any other food group

EDIT: I think most Americans would be shocked to discover that their "favorite" food group is added fat/oil.