I feel like the time demands of modern life are the main culprit here. Today families mostly need two working parents, which means nobody to take care of the baby full-time.

If you have someone taking care of them full-time, toilet training early is usually easy and a net time save.

But if you can't ever invest that time because of the time version of the poverty trap, you are in diapers until the kids are developed enough to make the transition themselves or by seeing other examples (at daycare, etc), or are just old enough that it can be explained to them.