TL;DR: Japan doesn't have ubiquitous public garbage cans - you are expected to take your garbage with you until get home.
It's more than that. Kids are expected to clean their schools, even the bathrooms. Not surprisingly, this nurtures an attitude of not messing it up in the first place.
Ever go out at 3am? Rats and trash everywhere.
Squeaky clean by morning though, their cleaners are amazing.
Lack of trash cans is because of sarin attacks, nothing else.
So they just put the trash in the street.