I think the leading cause has been the eroding of community from our societies. Air conditioning, TV, and now the internet. The decline in memberships to clubs. The decline in religious attendance. People don’t know their neighbors anymore. For some this causes stress, but for others they just miss the wave of social interaction they would have been swept up in because that was how life was. You were in the bowling league at work, you attended church, and your network was nudging you to settle down and pair off. You probably always knew who your potential mates were anyway, because you knew their names from kindergarten.

Now there are a lot more options and a lot more distractions and less of the kind of social interaction that leads to marriage and children.

It’s a global phenomenon. Look for “demographic winter”.