I’m not sure what the context was around this question but I think that “What did you have on” vs. “What were you having on” are both acceptable for asking this question. They most likely chose to say “What did you have on” because it’s asking in past tense and sounds better than asking “What were you having on”. There are multiple ways to ask questions, and say phrases. Most of the time it really just up to the writer, in your case of your english book to pick which they want to use.