Payment methods vary by country and some payment methods might not be available with all subscription types (family, student, etc).
Since the student discount just launched in more countries, it could be that credit card is the only option right now for those countries and could expand to more options in the future. There have been other users posting here that the only option it gives them is credit card. So I'm not really sure there is not much information on this yet. It's still very new.