There's a plateauing effect that's been observed when health, happiness, and comfort are reached within a country. When your country doesn't have a high infant/child mortality rate, you don't need to have more than 2-3 kids. So the birthrate tends to go down.

Japan is sort of a whole different beast. There's some pretty good documentaries that discuss the issues when observed from certain fronts: the grind culture, lack of childcare, lack of gender equality.

There's a lot of cultural things that need to be fixed in Japan, and no one wants to change.