And there something distinctive about Korean culture that made it more receptive to the endeavors of Christian missionaries?
While there's much more to be said on the topic, here are some earlier answers that may be of interest to you:
Why did Christianity grow in South Korea after WWII to the point that now a third of the country is Christian, while Japan hasn't risen above 3% Christian--considering that both countries had to rebuild themselves from scratch after the war(s) and came under heavy American influence? with answers by u/ivymikey and u/dsk_oz, and
Why was Pyongyang once referred to as "Jerusalem of the East"? by u/wotan_weevil.