Different circumstances. SK and Taiwan was forcefully decolonized and then built up to buffers to communism by the US.
Congo got screwed over by Belgium.
South Korea was a military dictatorship and economic backwater for a very long time. We think it was more successful in hindsight given how crazy things got under Kim Jong Il in the North and how South Korea now is a stable democracy and has been for most of our life times, but that's mostly a product of the 80s and 90s.
North Korea had a higher per capita GDP than South Korea until the middle of the 1970s, for example. That was not so much due to the nature of their governments (though that didn't help) as it was a combination of that and the existing infrastructure on the ground. Most of the previous industry and state apparatus of Korea was in the North, while the South was primarily agricultural hinterland that had to be built up (after the dictator enriched himself, crushed all opposition, and so on, of course).
Congo is a good example of that kind of problem. A longer decolonization would not have solved things, it would just be Belgians enriching themselves more, but more foreign aid spent in the training and construction of political, administrative, and industrial infrastructure was probably necessary to avoid the type of collapse we have now.
That aid would not ever come from Belgium. It was pretty much only conceivable via the US or the USSR, and Eisenhower made it very clear how jumpy he was about communism (though, to be fair, Lumumba had a habit as a former postal worker without much government experience of being a bit too reactive and alienating the political class of other nations).
Quite frankly, the best and most likely path for the Congo is if Lumumba manages to avoid alienating a couple less people internally or externally, but is aligned enough with the USSR that they start backing him and the Congo to the hilt while the US focuses its energies in other parts of the world.