Japan occupied Korea from 1910 until 1945, at which point administration of the country was split between the USA in the South and the USSR in the north. Following an indecisive war lasted from 1950-53, the Korean peninsula ended up witht he situation we see today. https://en.wikipedia.org/wiki/History_of_North_Korea

China's history is more complicated, but is also the product of military conflict following colonial imposition by Japan. To suggest that either country's political system is the outcome of inherent structural weaknesses in multi-party democracy is absurd.