Comment by prawn

Here's my related guess: many people want to wait until they feel in an appropriate position personally and financially before they have children. That might mean earning a certain amount or feeling like they have a foothold in home ownership or waiting for a future renovation. Only, the world in 2010+ constantly exposes you to bigger options on top of the issue of housing affordability: more adventures, more expensive furnishings, stories of others' salaries, etc. It would be quite easy to push back child-rearing for 5 or more years if you didn't feel like you were quite where you wanted to be, or wonder if you'll ever get there.