There are some theoretical explanations for why Christianity has gained such a foothold in South Korea when it has failed to gain much traction in other East Asian countries. One of them is that the message of salvation resonated very heavily with Koreans during the occupation of Korea by the Japanese. Another is that the hierarchical culture of Korea is very congruent with the Christian beliefs and practices.
I think the most important reason, however, is that Christians were heavily involved and visible in the independence movement as they were severely persecuted by the Japanese during the occupation. Some actions that the Christians took that solidified their link to the independence movement include the refusal to worship the Japanese emperor as a god and a small majority of the signee's of the March 1st Movement were some form of Christian.
For more information, click here.