In a post from a few years back, Megan McArdle explained why the North American diet of the mid-20th century was so dreadful (at least to a modern-day foodie):

Here are my prime candidates for why I think they ate like that:

1. Most people are not that adventurous; they like what’s familiar. American adults ate what they did in the 1950s because of what their parents had served them in the 1920s: bland, and heavy on preserved foods like canned pineapple and mayonnaise.
2. A lot of the ingredients we take for granted were expensive and hard to get. Off-season, fresh produce was elusive: The much-maligned iceberg lettuce was easy to ship, and kept for a long time, making it one of the few things you could reliably get year round. Spices were more expensive, especially relative to household incomes. You have a refrigerator full of good-looking fresh ingredients, and a cabinet overflowing with spices, not because you’re a better person with a more refined palate; you have those things because you live in 2015, when they are cheaply and ubiquitously available. Your average housewife in 1950 did not have the food budget to have 40 spices in her cabinets, or fresh green beans in the crisper drawer all winter.
3. People were poorer. Household incomes grew enormously, and as they did, food budgets shrank relative to the rest of our consumption. People in the 1960s also liked steak and chicken breasts better than frankfurters and canned meats. But most of them couldn’t afford to indulge their desires so often.

The same people who chuckle at the things done with cocktail franks and canned tuna will happily eat something like the tripe dishes common in many ethnic cuisines. Yet tripe has absolutely nothing to recommend it as a food product, except that it is practically free; almost anything you cooked with tripe would be just as good, if not better, without the tripe in it. If you understand why folks ate Trippa alla Romana, you should not be confused about the tuna casserole or the creamed chipped beef on toast.

4. The foods of today’s lower middle class are the foods of yesterday’s tycoons. Before the 1890s, gelatin was a food that only rich people could regularly have. It had to be laboriously made from irish moss, or calf’s foot jelly (a disgusting process), or primitive gelatin products that were hard to use. The invention of modern powdered gelatin made these things not merely easy, but also cheap. Around 1900, people were suddenly given the tools to make luxury foods. As with modern Americans sticking a flat panel television in every room, they went a bit wild. As they did again when refrigerators made frozen delights possible. As they did with jarred mayonnaise, canned pineapple, and every other luxury item that moved down-market. Of course, they still didn’t have a trained hired cook at home, so the versions that made their way into average homes were not as good as the versions that had been served at J. P. Morgan’s table in 1890. But it was still exciting to be able to have a tomato aspic for lunch, in the same way modern foodies would be excited if they found a way to pull together Nobu’s menu in a few minutes, for a few cents a serving.

Over time, the ubiquity of these foods made them déclassé. Just as rich people stopped installing wall-to-wall carpeting when it became a standard option in tract homes, they stopped eating so many jello molds and mayonnaise salads when they became the mainstay of every church potluck and school cafeteria. That’s why eating those items now has a strong class connotation.

5. There were a lot of bad cooks around. These days, people who don’t like to cook, or aren’t good at it, mostly don’t. They can serve a rich variety of prepared foods, and enjoy takeout and restaurants. Why would you labor over something you hate, when someone else will sell you something better for only slightly more than it would cost you to make something bad? […]
6. Look at the sources of our immigrants. Immigration is still the major way that countries get new foods (if you don’t believe me, go out for Mexican food in any European country and report back). With the notable exception of the Italians, in the 19th century, most immigrants were from places with short growing seasons and bland cuisines, heavy on the cream and carbohydrates. After we restricted immigration in the 1920s, that’s what we were left with until immigrants started coming again in the 1960s. Of course, Louisiana had good French food, California and Texas had a Mexican influence, but by and large what we ate in 1960 was about what you’d expect from a German/English/Irish/Eastern European culinary heritage, adapted for modern convenience foods. And people liked it for the same reason I like jello salad: It’s what they were used to.
7. Entertaining was mandatory. Because people didn’t go to restaurants so much, they spent time having people over, or eating at someone else’s house. If someone had you over, you had to have them over. This meant people had to have “company dinners” they could make, or at least a stock of canapés they could throw together for a cocktail party, even if they weren’t very good at it. Cue the weird focus on prettying everything up, more than occasionally to the detriment of the food itself: if you can’t make it good, you can at least make it pretty, to show people you made an effort.