The main reason is the people and culture of those countries.

Both Koreas are ethnically and culturally the same. What makes one a SF near-utopia and the other a starving disgrace is the accident of having fallen within opposite spheres of influence during the Cold War and the subsequent development of radically different political systems. One could argue something similar happened with pre-unification Germany. I've read somewhere that the relative poverty in rural Southern Italy and wealth in industrial Northern Italy mirror the North-South dynamics of Reconstruction USA.