Japan currently has two space launch facilities: Uchinoura Space Center (31°15′07″N 131°04′55″E) and Tanegashima Space Center (30°24′00″N 130°58′12″E).
Both of these are at the south end of the main archipelago, but Japan has access to the Okinawa islands (around 26° 19' 34.80" N) and the Yaeyama Island group (around 24° 21' 28.19" N).
Latitude elevation is an important factor when launching orbital boosters since the earth's rotation helps a lot, and since it reduces the need for maneuvers for geosynchronous orbits.
Six degrees closer to the equator sounds like a very large difference to me.
Why did Japan not build the space centers further south?