There is also the reality that before WWII, much of North America was rural - the majority of people lived in small towns and/or on farms. Post-WWII the North American economies BOOMED (well, Canada and the USA did - not sure about Mexico) and people en masse moved into the cities and away from hard manual labour. Instant, processed, and ready-made/pre-packaged foods were also developed as part of the war effort and started to appear in greater numbers on grocery store shelves after the war ended, fast food restaurants proliferated and many more women entered the work force, so there was more of a market for pre-packaged food. Overall, North Americans in general and Americans in particular have been the victims of their own economic and social success, with respect to the overall health of the population(s).
Another shift that has occurred as the industrialization of western economies in general has matured is one away from manufacturing (and manual labour) and production and toward service industries.
These are genearlizations, of course...