Because "o espectador" is a singular thing and "audience" implies a crowd (unless we are being facetious). It could be, "audience member" or "member of the audience" but that still implies there is a crowd around the, "espectador" and not that he is alone (and, I doubt either of those are accepted by the limited Duo computer).
That makes sense.
My question comes because 'spectator' isn't a word that is commonly used in the midwestern united states. So while spectator is correct, it sounds awkward to me. I've never heard spectator used outside of references to sports. I would refer to an audience or a crowd of people at a concert or show, but I would never call them spectators.
We do speak of spectator sports (sports that people watch, as opposed to sports people play without an audience). Though these days, I guess even snowshoeing or ice fishing could be spectator sports, thanks to the omnipresence of the internet. But there was a time when only some sports were. ; )