Probably general decline of meal culture--IIRC home-cooked meals went from ~90% in 1900 to ~70% by 1950 to ~50% in 2000, and the definition of "home-cooked" changed a lot. Science could do no wrong after WW2 so folks didn't question the rise of frozen meals and miracle ingredients.
About oils, this image starts in 1961 but you could extrapolate backward a bit. Also:
Lots of: urban hiking, cycling, sprinting
Lots of: fresh meat, seafood, eggs, organs, tubers, starch fruits, vegetables, meat fat, dairy fat, oil fruits
Some: cured meat, dairy protein, sweet fruits, rice, pulses, tree nuts, oil seeds
Minimal: soy, refined proteins, sugar, liquid carbohydrate, grains, refined oils, peanuts