High population density, like many of the other low birth rate nations. At some point people don't thrive living so close and crowded together. The carrying capacity of civilization has its limits.
That’s a really interesting observation. This would presumably drive a measurable difference within the same country between the high-density cities and the rural areas. I wonder if stats are available on a per-locality basis anywhere?
There's no country with people primarily living in high-rise buildings where TFR is measurably above 1.