possible contributing factor.

other factors are likely:
1) mask usage and social distancing are more culturally ingrained in Asian countries from previous respiratory virus outbreaks
2) Possible greater levels of immunity from prior exposure to other coronaviruses
3) more socially distant cultural norms (no handshakes, hugs, kisses for greeting), although you could argue this is offset by the population density of Japan
4) More acceptance of government intrusion in tracking outbreaks and enforcing quarantines