The reason mainly lies in the different political ideologies and concepts they adhere to. While North Korea follows communist ideals followed by a dictatorial government system, South Korea adopted a democratic political system with an open capitalist economy.
Moreover, the two countries were a part of the Cold War as well. The democratic West supported the South while the Communist east led by the Soviet union aided North.
He called the imaginary barrier the "Iron Curtain". He used this image to oppose the Soviet union expansion, and keep in communism in check. Hope this helps!!!:)
