Japan, South Korea, and Taiwan were all once brutal authoritarian regimes. However, while these three have largely made the successful transition to democratization, North Korea and the PRC remain mostly authoritarian. Why didn't North Korea and the PRC undergo this same process?

To further clarify, I am not asking for why the Democratization efforts in Japan, South Korea, and Taiwan succeeded, which there are ample readings on, but specifically why they failed in North Korea and the PRC. Many of the reasons given for the previous nations, whether cultural, economic, social, the brutality of the regime, etc. etc., would seem to apply to the latter, particularly with the Koreas, yet the same transition has seemingly not happened.

Apologies if this breaks the politics or 20 years rule