A few generations of wild economic growth and thus less of a need for kids to look after their parents, shifting norms, and lag time in norms shifting back, perhaps. Though I don't really know if this fits.
But my conditions for having kids were only if they had significant security in quality of life and I would be able to spend time with them. I only had kids because my household income was at a level I deemed so, which is significantly above the median. I actually would have been fine with no children, but my wife wanted to experience being a mother.
The big difference in recent decades compared to all of human history is that today, people can have sex without risking having children due to very effective and convenient forms of birth control.
Edit: And women have financial independence so they can decide whether or not to have children on their own.