It's because '구' in "누구" doesn't have a final consonant. e.g. 선생님 + 이(다) + 시 + 어요 ☞ 선생님 + 이 + 시 + 어요 ☞ 선생님이세요? ("세요" is short for "시어요") ('님' has the final consonant 'ㅁ', so '이' added before "세요") You can say either "누구세요?" or "누구십니까?" without '이'. \^o^/

It is because 세요 already has embedded 이 in it. we always drop 이 before the honorific 시.