Japan has no history of gun culture so that's why. But it doesn't stop people, especially Yakuza from shooting people or using grenades. Yes, quite a few years ago there were signs warning about "drive by grenadings" between Yakuza families. People just strangle people or stab people if they want to kill them. If you ever want to know about all the fucked up shit that happens in Japan just go to the website Tokyo Reporter (NSFW).