I think it related to the general availability of contraception and that the work involved in caring for children has increased, while the number of carers has decreased.
The trend toward migration means grandparents are often not nearby to help care for kids. This coupled with the fact that in most cases both parents need to work increases the difficulty in raising kids dramatically.
I’ve also noticed that, while grandparents are not around to help, fathers often still don’t involve themselves in childcare.
All this coupled with how expensive kids are means that if you can satisfy yourself with one child, people often do. Enough that it depresses the birth rate significantly in any case.