Generally it's attributed to the time around WW2, which for Americans included the effects of rationing as well as being exposed to prepackaged foods while deployed. Throw in a bunch of marketing and nationalism and the breadcrumbs start to line up. https://www.sciencehistory.org/stories/magazine/from-the-fro...

If you really go down the rabbit hole, you start to see how many of the foods that baby boomers grew up on were first fed widely to the parents during the war.