I like the explanation here by Hans Rosling - We'll settle around 11 billion people

https://www.youtube.com/watch?v=2LyzBoHo5EI

TLDW: People in the past had more kids more often because of high mortality rate and having a care-taker (mother) at home.

With child mortality being lower, plus women entering the workforce and enjoying a higher standard of living, most people won't want to have more than ~2 children. They want to spend their money on having a higher quality life rather than subsistance and survival.