The socio/ cultural/ historical part of your Q is too big to answer concisely (why here and not elsewhere)... but if I were researching a paper on it... here would be the brainstorm on it:
- Neurology (as in we know know how the sympathetic and parasympathetic system works in regards to elimination... and that it's not willfulness to be spanked out, but basic physiology). So there is solid science behind the fact that some children are trained as young as walking and others not until 4ish. AKA it's not causal to parenting, but rather causal to neurology.
- Preponderance of dual income to stay at home parents (and absence of governesses)
- Cultural unacceptance of 1/2 naked children
- Number of siblings
- Style of toilets (Western v Asian... asian toilets are a natural style which lines up with the anatomy of our bodies... aka squatting, whereas western toilets. It is FAR easier for children to train on asian toilets, because there isn't a lot of pain and rewiring of the somatic nervous system - aka we use different muscles when squatting v sitting).
- Technology (disposable diapers OR washing machines & diaper services v. old method of scraping / washing / bleaching/ boiling on the stove for HOURS). What plays into what? Since women had more children was it just a fact of life to be scraping and boiling for years or something to be done with as quickly as possible? Or since disposables are expensive, and our level of expected manual work (electric irons, dishwashers, washing machines) so much less does the added expense and work play into wanting to train earlier or does it make it easier to stay in diapers longer? Which way is which?
- Willingness of parents to become "trained" (aka the number of children who are NOT trained, but whose parents say they are and spend all day every day whisking them in and out of the bathroom for several months to a couple of years... instead of the child heading into the bathroom, relieving themselves, wiping, flushing, and washing their hands with help only to reach tall appliances or wiping their bum - which short arms can't reach).
- Definition of "trained". I know 2 families whose 2 yo are completely trained (just need help wiping and washing, but self monitor), but I know HUNDREDS of families who ask the child, or even lead them, every 30 minutes into the bathroom... and who will 'accident' all day long without that assist... who call their children trained. I also know far far more families who don't consider their 2yos trained, because they would accident all day without assist. If 2 children at age 2 both accident without adult monitoring until age 3, and one familiy says their child trained at 2 and the other 3... but they're both doing the same thing... that's a problem.
Those would be what I would start out with, and then researching globally to find out how different cultures, countries, and historically children are trained would be the complete answer. It's a big question.
In my family: I and one of my brothers was trained at 18mo-2years. Another of my brothers was trained at 4.5 (mum was getting worried about kindergarten with him), but most trained around age 3. As did my own son. For all 40 of us (sibs, cousins mine, cousins son's) the average time was about 1 week for daytraining, and night training in also about a week 6mo-1 year later. But NONE of us were pushed, our parents and myself, we just followed family history of waiting until each child was ready... instead of when we wanted them to be ready.
((Our definition of training is 100% self monitored, only needs help wiping/washingup... perhaps 1 accident a month tops)).