Until recently Japan had very few foreign nationals, on the order of 1% even in 2005 [1]. Compare that to 10-12% in the US. That has recently been changing as Japan tries to bring in more foreigners to address its shrinking workforce [2].

[1] https://en.wikipedia.org/wiki/Demographics_of_Japan#Foreign_...

[2] https://foreignpolicy.com/2020/06/23/japan-immigration-polic...

Homogeneous island nation! Like 97% of people in Japan are Japanese.

https://en.wikipedia.org/wiki/Ethnic_groups_of_Japan

Japanese call it "Galapagos syndrome"