이름이 뭐야? We can use it when starting a new question. e.g. A:이름이 뭐야? B:이름은 XXX야. A:직업은 뭐야? B:OOO야. 이름은 뭐야? We can use it when continuing other questions after one or more question. e.g. A:직업이 뭐야? B:XXX야. A:이름은 뭐야? B:이름은 OOO야. From http://www.westudykorean.com/particles1.php -은/는 is a topic particle that marks the topic in the sentence in order tell us what the sentence is about. Examples) 1. 벤은 강아지에요. As for Ben, (he) is a puppy. 2. 앤은 학생이에요. As for Ann, (she) is a student. -은/는 is also used to change the topic. e.g. 1. 어디 갔었어? 시험은 어땠어? Where have you been? How was the exam? (Now the topic is the exam) 2. 그 아파트 어때? 가격은 얼마야? How’s the apartment? How much is it? (Now the topic is the price of the apartment) When -은/는 is NOT in the beginning of the sentence, the sentence is used for comparing and contrasting. e.g. 1. 연필은 있지만 볼펜은 없어. I have pencils, but I don't have pens. 2. 수지는 얼굴이 예뻐. Susie's face is pretty. 수지가 얼굴은 예뻐. (하지만 성격이 너무 나빠.) Susie's FACE is pretty. (but her personality is really bad.) To talk about the past information. e.g. 1. 여행은 누구랑 갔어? Who did you travel with? 2. 이 책은 어디에서 찾았어? Where did you find this book? When brining up a new topic to the conversation, -이/가 (marking the subject) is used for questioning, and -은/는 (marking the topic) is used for answering the question. e.g. 1. 전공이 뭐예요? (major '전공' is the subject of the sentence) What is your major? 전공은 미술이에요. (the topic is changed to a major) (My) major is art. 2. 생일이 언제예요? (birthday '생일' is the subject of the sentence) When is your birthday? 생일은 9월 6일이에요. (the topic is changed to a birthday) My birthday is September 6th.

I would say it's because you are simply asking about something that is the subject in the sentence (이) not drawing attention to a new topic (은). :)

I think "이름은 뭐야?" is more like "what are names?" as a concept.