Most of the video of Lee Mujin in Youtube don't have any English Subtitles, unless you can understand korean then you would probably understand why he became a very well known artist there. For people who are curious, here is the summary of some facts about his popularity. Thank you ^^
LINK :

1. LEE MUJIN'S VIRAL VIDEO in Seoul of Institute of the Arts -    • 서울예대 그 유명한 복도  
2. LEE MUJIN SING AGAIN AUDITION -    • ♨핫클립♨ ＂난 노란 신호등 같은 존재.＂ 특이한 음색으로 최고의 ...  
3. TRAFFIC LIGHT (Official MV) -    • [MV] Lee Mujin(이무진) _ Traffic light(신호등)