Massive family planning campaigns, heavily funded by western countries, to change the culture around having children. (My dad does exactly that for a living.) In Asia you now have multiple generations of people who associate having big families with poverty and backwardness.
Sounds like culture doesn't mean much when even the lands where the culture originated from willingly discard the cultural elements that lead to more children and larger families, even while preserving other elements.
Consider the case of the Islamic Republic of Iran:
rayiner|2 years ago
exBarrelSpoiler|2 years ago
Consider the case of the Islamic Republic of Iran:
https://fred.stlouisfed.org/series/SPDYNCBRTINIRN
https://www.dw.com/en/iran-birth-rate-decline/a-54371973
So much for white Americans and westerners.