We've all seen old-timey recipes that include copious amounts of canned olives, hot dogs and jello, crushed graham crackers, bag marshmallows, canned meat, and any number of other ingredients that modern consumers would regard as disgusting. I think I Intuitively understand why these types of ingredients would have appealed to Americans when they were new, so what I don't understand is why there has been such a radical change, and a strong emphasis on freshness, farm to table, and whole ingredient cooking.
It took two things to bring about a focus on 'fresh' and 'whole' foods: availability of ingredients, first, and second, the willingness to put them to use. I'll address the second of those first (and I would note at the outset that this focuses on middle-America with forays to France and the UK. There's a whole world out there beyond this narrow focus that the below isn't meant to denigrate or pass over.)
James Beard, Julia Child, and Craig Claiborne, and Jacques Pepin, along with broadcast TV and expanded travel can all be reasonably said to have brought about a focus on 'modern' food in the US. If you read through Mastering the Art of French Cooking (1961), you can see the transition. Julia has recipes for eggs in aspic, but the cookbook is decidedly reliant on fresh ingredients - one does not make a salad nicoise without access to good, fresh food. So too, The New York Times Cookbook came out in 1961, and it focuses, significantly, on what were then modern American recipes, with some cultural forays, and a real reliance on the availability of fresh fruits and vegetables. You can cook from either book today, but you'll need a trip to the grocery store for many of the recipes. What is material is that you can get the ingredients you'll need from the grocery store. Both books were significant in bringing a breadth of recipes (including ingredients and techniques) to more families.
But I would suggest that if you look back a little further, the roots predate WWII by just a little. James Beard wrote his first cookbook in 1940, and he had a TV program - "I love to Eat" - starting in 1946. Beard's cooking and experience in food are all what you'd generally consider to be 'fresh' and 'whole,' with an Americanized French technique and a conscious effort to bring fancier techniques and ingredients to more people. His experience cooking around and predating this time period focused on French techniques and preparations. Looking through his first cookbook, Hors D'Oeuvre and Canapés, you'll find recipes that, while outdated, are certainly a league apart from jello molds and canned cocktail wieners.
It's one thing to have modern recipes in a catering company in New York and quite another to have them elsewhere, so I would suggest that Beard's first effort - particularly judging by sales figures - is not really indicative of a national trend. But comparing Hors D'Oeuvre and Canapés to Beard's American Cookery (1972) you'll see that (a) not everyone everywhere was eating jello molds until the 1970s, but (b) the country had come a long way over the decades. In fact, looking at menus at, say, Delmonico (1827, New York) or Antoine's (1840, New Orleans), from more than 100 years ago, there were a number of what you'd probably call 'whole' American foods: wild duck, fish, and turtle, for instance, much of which was transported well across the country. The roots were there, but not the flowers, because there was little availability
So what changed? Shipping, containerization, and refrigeration. With the growth in containerization (1956) and then insulated shipping (1960s) and refrigerated shipping (1970s), more fresh and whole foods became available to more people. Chez Panisse, which is credited with sort of the apotehosis of 'fresh' and 'whole' foods opened in 1971. This is not just true in the US. In France, Jacques Pepin discusses his first experience with an alligator pear (avocado) after WWII. (Thinking it was a pear, he bit straight through the skin, to his dismay.) Avocados were not really available in France prior.
But it's definitely possible to over-tell this story. Looking at the food James Beard would have consumed in Portland, Oregon in the 1920s, you'll see a lot of overlap with Chez Panisse down the coast some forty years later: fresh fish, for instance; fresh fruit; fresh vegetables. The reason is in large part that California and the Pacific Northwest are, well, pretty good for growing things. David Chang once complained that "Fuckin’ every restaurant in San Francisco is just serving figs on a plate. Do something with your food." Why? Because the fresh figs are there, and fresh, and local - unlike New York.
The thing that containerization / refrigeration changed was allowing more fruit and vegetables to make it to more of the country. In 1975, very few vegetables and about 20% of fruit was imported. By 2015 it's about 50% of fruit and 25% of vegetables, and growing. There's a concomitant rise in consumption of fruit and vegetables, and in interest in how to prepare fruit and vegetables, and part of this is the rise in per capita income in the US. Julia Child et al were there to show Americans how to to prepare these foods.
You asked about 'farm-to-table.' Farm-to-table cooking has been an undercurrent of US cuisine, and cuisine more broadly. That we're eating more imported fruits and vegetables rather than fewer is strong evidence that it remains an undercurrent, not a dominant trend. It coincides with a discovery, or I guess rediscovery, of local food: to read the NYT cookbook from 1961 is an adventure. It has a gumbo recipe without a roux, and most of its local recipes are similarly, well, hardly recognizable to those who would originally make them. But the trend in restaurants toward locality really never stopped existing. It's had a bigger focus at different places in different times - a lot of French restaurants never - I would put it in the mid-1990s with St. John in London, which was reasonably revolutionary in the food world - just simple, local food prepared simply. Why did it happen? I'd say that in one part it's the natural progression: once you have everything you need to make all Julia Child's recipes in the grocery store, you are probably going to be interested in pushing boundaries. A focus on the chef is part of a focus on the artisan, and what are (local) farmers and butchers if not artisans?
Sources:
The Man who Ate Too Much (Birdsall, 2020)
The Apprentice (Pepin, 2003)
Mastering the Art of French Cooking (Child, 1961)
Ten Restaurants that Changed America (Freedman, 2016)
The Perfectionist: Life and Death in Haute Cuisine (Chelminski 2006)
Dominion from Sea to Sea: Pacific Ascendancy and American Power (Cummings, 2009)