Earlier quoted context omitted.
Considering the physical proximity of Japan and China (30% of tourists to Japan are Chinese) the fact that Japanese hospitals haven’t been overwhelmed like Italy suggests the virus is not spreading as rapidly there. (Testing may also be a concern but lack of testing cannot hide overwhelmed hospitals).
But why? I have pushed my air quality hypothesis. What other reasons are there. And you can't just claim social distancing. Unless they have shut down their trains, where people are literally packed in, that just doesn't fly. (So, have they?)
Second explanation that comes to mind is that they tend to be just healthier than other countries. They have low obesity, exercise correctly and routinely, walk and ride bikes to get where they go, and do all of this even when elderly. The counterpoint to this is that they have a really high smoking population.
Source: I lived there for two years.