Because China has adequate and well trained police forces that take women reporting sexual crimes seriously, particularly in urban areas where most rail traffic is. Along with that they actively patrol and monitor stations and trains for any illicit behavior. They will then beat the shit out of perverts and rapists before throwing the book at them.

In countries like Japan and India, police forces rarely listen to women, and even if they do it’s just an annoyance to them and they will do no follow up. This leads to extremely high rates of sexual abuse, even in public, as perverts and sexual criminals know they can get away with it. So segregated train cars become a necessity.