Because opposition, whether armed or otherwise, has been systematically repressed by the US-backed Liberal Democratic Party. Japan has effectively been a one-party state since the occupation began. Like, there are members of the Japanese Red Army, United Red Army and
East-Asia Anti-Japan Armed Front who're still alive, in prison. The rest have all either died of natural causes or await execution. I suspect the JCP has only seen a surge in membership strictly because the JSP collapsed in 1996, and the SDP sucks ass.