It was because aspics were meant to 'contain' stuff, not just be gross, savory Jell-O. Like, a traditional aspic salad would have various fruits suspended in it.

I think when gelatin became common in grocery stores, people were just all about the novelty. If you read cookbooks from the 50s that have these recipes in them, you see a commonality — people were just chuffed as chips that they could make a cake that jiggles lol.