I know it’s not the question you asked, but I don’t think there is likely to be 1 reason. Our society is so complex and diverse that this and every other socioeconomic indicator is the confluence of dozens of different factors and forces that interact in non-obvious and non-linear and mostly unobservable ways.

Look at all the explanations being suggested in this thread. Which one is right? Maybe all of them! Each on their own contributes a little piece of the puzzle, adding up to the effect we see.

This myth of monocausality leads to very bad policy decisions, where people jump on an explanation as the One True Cause of the Current Thing. It’s oversimplifying and polarizing and moves us further from truth and progress.