The reason for cancellation...try to tell me I'm wrong on this
The problem is NOT the number of groups, judges, format, or any other items you can come up with. The show didn't make it because acappella is not a hot music genre. PERIOD. It's the same reason you don't see "next broadway star" or competitions like that. Idol and XFactor are there to produce the next Nicki Minaj, Mariah Carey, or Daughtry. (Phil Phillips not included in this rant).
If it's not auto-tuned, overly edited, over produced garbage it no longer appeals to much of society and that's why the ratings aren't very good. It's sad, but no-one really appreciates a good harmony these days. Truly sad.
I want this show to continue as there are so many acappella groups out there that deserve to be showcased, but sadly it probably won't happen.
As a sidenote - Many people think/thought Glee was going to revive acapella, which I also thought would be true during the first season. When their original version of "Don't Stop Believing" was creating a buzz it seemed like this could put acapella at least back on the radar of the general population. But then Glee introduced backing tracks and all the other stuff that we hear on the radio everyday and it was just like listening to the radio.