For me it’s primarily time & lack of family close by - mainly driven by both parents having to work & moving away from home area for job. Financial aspect also a consideration but not a showstopper - taken me many years to reach a comfortable level of financial independence however. Many of my colleagues in a similar position & have 1 or 2 kids max as a result. Not enough hours in the day.
Discussion
I think it's people sleepwalking their life. the developed world has clear boundaries, every step is basically regulated. if you let people figure things out as they go, they'll want kids again, it's the only things that comes unprepared, nobody's ready for it before it arrives. that's the last unregulated thing in our world, it's a true jump in uncharted territory.