Immediately after WW2 the Soviet Union occupied the North, and the United States the South which led to an growing ideological shift between the two countries as the occupiers had a lot of sway. Both the North and South Koreas did hold democratic elections in 1948, and unsurprisingly a Communist won in the North aligned with the Soviet Union, and an anti-Communist in the South. Kim Il-Sung won in North Korea, and remained in power until 1994. It was with this long rule where the North Korea we know today was molded.

Technically North Korea is democratic, and they still are holding elections every 4-5 years. It is just that the winner has been Kim Il-Sung or his descendants for 70+ years straight, and of course that brings into question the validity of the elections. While it is considered authoritarian, North Korea isn't actually a hereditary Monarchy as it may appear to be.

It's also important to remember that South Korea wasn't always consistently democratic either, and it wasn't until 1980 when their current Democratic process began (Park Chung-hee ruled as a military dictator from 1963-79). By that time North Korea's cult of personality around Kim Il-Sung was already fully indoctrinated in North Korea.