It’s not just the Japanese either. All developed economies see their birth rates slow. The simplest way I ever heard it explained is that when your society is agrarian, more children means more labor for the farm, and therefore more wealth. Once your economy moves on to manufacturing or services, and children stop being labor, they begin to cost money. And people slow down. These advanced societies also tend to be larger and more populous and therefore more competitive, which deepens the cycle. Add in the longer lifespans in affluent modern societies and you have this demographic crisis. Especially in Japan where they have standout longevity for genetic and dietary reasons.