I'm referring to South Korea as occupied Korea and North Korea as independent Korea.
Korea was divided due to US invasion and occupation that never ended. The war never ended and cannot end without the United States signing off, which it refuses to do. Massive US bases remain in South Korea and it is not really a sovereign country, it is controlled by Chaebols and similar monopolies entrenched with US-dominated capitalism.
North Korea, in contrast, remains independent. It outperformed the South until the fall of the USSR, after which the US-led capitalist faction of nations proceeded to isolate it, cutting off fuel and other kinds of trade on which it depended. Combined with natural disasters, this led to the near famine conditions of the 90s that are what the false memes about North Korean poverty often refer to. The philosophy of Juche strengthened as a result of this experience and North Korea is now much more secure and independent, though it has close economic ties to China.
Incidentally, the US invasion was essentially on the pro-Japanese side. The (more popular) socialists rejected the Japanese imperialists as well as their collaborators while the US embraced both, basically just stepping into Japan's shoes because they could use the existing apparatus to mass slaughter pro-independence people in the South.