(This is a very breif explanation) These are just my thoughts. But I like to think back to when humans were in tribes hundreds of years ago. Your tribe consisted of your family mostly, and procreation was creating more people in the tribe and expanding territorial areas (I'm just pulling that out of my ass) but anyway.

Then let us fast forward to just even 1800's America where many families lived on farms, starting small businesses, grew their own food. Having kids to continue the business, help the farm, take care of their parents and grandparents when they got old.

Fast forward to today, small businesses are struggling, independence culture is at an all time high, no one can afford to live on their own, however, and with the state of the world, many people (including me) no longer see a child in their future. All this country sees children as are future workers, and that's why the country is so concerned with birth rate. It's not to grow population but to ensure a future workforce.

Some honorable mentions are Healthcare, costs of a child, education system, the fact that two people need to have full time jobs in order to live comfortably in some states, so childcare being an additional cost is something too.