The real reason behind all the gelatin salad abominations is that after gelatin was first discovered/isolated, it was very costly to produce, but new technology made it much more affordable.

Isolating gelatin requires long cook times (which require lots of fuel) at ideally fairly low temperatures. Then there needs to be some level of filtration to make it as flavorless as possible, and then dehydration to sheets or a powder.

Finally, to actually make one of these "salads", you need refrigeration.

Production of gelatin was industrialized to make it much cheaper, and refrigerators became normal household appliances. You went from gelatin being only really used in "fine dining" to something you could do at home. In the same era, pineapple went from being a fruit that only the rich could get to something anyone could, so it went through a similar explosion of popularity.

The alternative funny answer is that the company that sold gelatin, Knox, was run by a husband and wife, and all the crazy stuff didn't start until the husband died, so either he was holding her back, or once she lost her husband, she thought everyone else should, too.