Japan is the quietest shard in League of Legends, and for a lot of players that is the entire selling point. The player base is small, the community norms are noticeably more polite than the Western shards, and the volume of abuse you absorb over a hundred games is a fraction of what NA or EUW will throw at you. If tilt is what ends your climbs, JP is worth the premium on its own.
Supply is the constraint. JP sign-ups are region-locked, so every account on this page is made by hand rather than generated in bulk — stock is thin but steady, and it is priced above NA for the same reason Korea is. The low population also means queue times stretch outside Japanese peak hours, which is worth knowing before you buy.
Note — JP sign-ups are region-locked, so every JP account here is hand-made.