It’s simply that for the first time in human history women have alternative options that don’t suck. That’s literally it. Birth rates will remain high only in socially conservative highly religious countries. Liberal democracies? No chance.
- larger percentage of population living in urban centers without the room for larger families
- full time employment for both spouses becomes necessary as cost of living increases
- delayed marriage and delayed age of first child means fewer children can be safely carried to birth
The "highly liberal democracies" is just pleasant coding for "consumerized".