I think audience can be both singular and plural in English. "Why are the audience" should be accepted, too.
Agree - two years on and it's not changed.
“Why are the audience not in the auditorium?” should be correct. The “correct answer” sounds awkward to me because I’m pretty sure “audience” is supposed to be plural. Reported.
"auditorium" is better be translated as "зрительный зал";
"Аудитория" is a lecture hall at the university, college etc.
"why are the viewers not in the auditorium" is not accepted, but "why aren't the viewers in the auditorium" is. Both should be, or neither.
Assuming it's really plural in Russian, why is it not genitive, зрителей, since they're not there??