I live in Japan. I guess you could argue that people are a bit more sexually repressed here (or rather, the objectification of women, especially high school girls, is worse), but I honestly think the subway groping problem mostly exists because of how crowded the subway cars get.
The main line in the city I live in is literally so packed during rush hour that you’re smushed up against everyone else, multiple body parts touching each other, unable to move an inch. If the women only car is full, I have to be smushed up against a dude. If someone groped me or took a picture up my skirt it would be very hard to tell who did it. Couple that with the Japanese tendency to not want to cause a fuss and stand out, and shaming over things like sexual harrassment, and gropers end up getting away with it.
To be fair, nowadays groping is taken VERY seriously and I’ve seen girls scream at guys/make a fuss on a few occasions. Also, a lot of men automatically grab the overhead bars with both hands to dispel worry from nearby women when the train is really crowded. Men’s rights activists are always arguing that the women only cars are “sexist” etc, but until groping stops happening gender segregated cars and loud camera shutters are going to have to be used.