I had a course on this topic recently, and I am curious to know the answer to the above question. The short story I got during my course was that becoming Christian was a way to "rebel" for a Korean, against Japanese Shinto rule, under the colonization (1905/1910-1945).. What other reasons were there?
Hi, while you're waiting for direct answers, you may find some information of interest, or at least users to ask follow-up questions of, in these threads
History of Christianity in the Korean Peninsula with some follow-up here How did South Korea become so Christian unlike surrounding countries like Japan and Taiwan?
Why did Christianity become so popular in Korea compared to other East Asian nations?
How and Why did South Korea convert so rapidly to Christianity?