Translation:Who am I?
I think I understood why; in 합쇼체 (Habsyoche) - (the conversational style generally called either the "formal" or the "formal polite"):
The Interrogative mood is used to ask questions. In 합쇼체, this mood is formed by adding -(스)ㅂ니까 to the verb stem.
When ending in a vowel the -ㅂ니까 is used. So in this case: 누구이 becomes 누구입니까 :-)