Well the trend started in the western world (US and western Europe) but is spreading across Asia and so on... The reason people had alot of kids back in the days, was because of all the diseases that could kill your children... So having alot was the best way of securing your family lived on, today - not so much.
So people are having less kids, so they have more time to work and more time to spend with each individual child.
If the trend spreads like these kinds of things normally do, it will eventually reach Africa as well
That and in the south it was free labor. For reals lol. My mom has 5 sisters and a brother. My GrandPaw has a lot of farm land. My dads side is the same.