I would say it's what consitutes adequate childcare combined with over reliance on parents as caregivers. So emphasis on nuclear family means you don't have grandparents, aunts, and uncles around so much. Paying for childcare is increasingly expensive as well. Even older children tend to be busier now with school and such which means that in a big family the older siblings aren't available to help out with the younger in the way that for instance my mother did as the third oldest of nine when she was a teen.
To add to that, people having kids later and having to move where career prospects are best means that fewer relatives are around to rely on, either because they’re too distant or have passed from old age.
My partner and I have three parents between us, and none are interested in helping with childcare as grandparents. My partner said that if she had known ahead of time we'd be entirely on our own, we wouldn't have had the two kids we did.
We get by with paid childcare instead, because we're here now and there is only one path forward.
