With development comes women's empowerment, and lower child death rates. Women started having fewer babies.
This is illustrated by how the birth rate dropped in the four "Asian Tigers" as they rapidly developed between the 1950s and the 1970s; today, these countries have total fertility rates less than 1.
Vietnam followed about 15 years later. It introduced a 2-child policy in 1988, but by then the birth rate had already fallen by nearly half. Because development.