This is exactly what you would imagine. Your application packet as a whole doesn't give them the information they need for confidently understand why you fit into the cohort. Perhaps your initial letter of recommendation has one of two common weaknesses sound with Korean applicants: the letter has minimal details, or it has no thoughtful assessment of the candidate's weaknesses, strengths and development needs.
This is exactly what you would imagine. Your application packet as a whole doesn't give them the information they need for confidently understand why you fit into the cohort. Perhaps your initial letter of recommendation has one of two common weaknesses sound with Korean applicants: the letter has minimal details, or it has no thoughtful assessment of the candidate's weaknesses, strengths and development needs.