When there is consequently more to do, more to experience, more people to meet, more to discover, more novelty to novel at. In short, the modern era has more of what makes the childhood experience what it is, enough to spill over into what was previously considered adulthood.

I sometimes ponder what would happen if we did discover the secret to substantial life extension or even immortality; if this phenomenon of trying to stretch that delightful halcyon glow of childhood wonder - what I've heard the Finns call lapsuus - would extend into decades or centuries or perpetuity as a result. It would if nothing else provide a plausible explanation for the behavior of characters like Q from Star Trek.

Children are massively more expensive, and less useful, in richer societies. I don't think anybody in Japan would be allowed to provide for their children the standards of care provided in a country with a growing population. As for the second point, if you are a farmer in a country with little physical mobility, your children are your retirement savings. If you live in America, your children are future employees of a company ten states away.

I read a statistic somewhere that it costs $2M to raise a child. Considering that it's considered a horrible imposition to ask your kids for money, or even to ask them to take care of you, it's easy to explain why urbanization, industrial development, and education, work against population growth.