There's a lot of economic explanations that seem perfectly legitimate.

I'm wondering if a simple contributor is the fact that many people are moving away from their immediate family. Then you feel more on your own when considering having child, which is significantly more daunting. I think a network of friends helps, but is simply not the same as parents/siblings/cousins sharing the load and advice. Let alone the experiences.

Also, it seems there's a negative feedback loop, where each person that chooses to postpone or not have kids influences their network to do the same.