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 :-)
Not sure where you're getting 누구이 from. Especially since 이 is the wrong subject marker to use anyways (it should be 가). The sentence is "저는 누구입니까?":
저는 = "I" or "regarding me"
누구입니까 = 누구 + 이다 = "who is"
It is a common formation for the noun (누구) to be attached to the copula 이다 when written.