I am thinking it was probably a bit of both. The U.S. at that time was “modernizing” with convenience foods. Fewer people cooked because more women were working than before. I am sure both class and culture play into the differences. The 1960s was a decade of huge change in the U.S. My parents explained to me the reason few people had gardens was because they did so much of that during the Great Depression, which is when a lot of the parents grew up.