You're right about the CMS. But unlike the Western ecosystem centered around WordPress, South Korea's public and web ecosystem is pathologically dependent on isolated local bulletin board system (BBS)-centric CMS platforms like 'GnuBoard' or 'ZeroBoard/XE.' As a result, when the government mandates censorship modules, it creates vendor lock-in, as those modules are supplied exclusively in the form of plugins for these local CMS platforms
so we don't have many if any of those vBulletin instances on Japanese Web either... That's just normal.