Ah 5v5 can be troublesome, so they simply do not include 5v5s in the matchmaking system. Or maybe they do, but there are never enough people in the pantheon queue for a 5v5. But even if you are matched against only AIs, I never saw anything bigger than a 4v4 in Pantheon.
I don't think that they're in at all, because you can play 3v3 on Crusible, so then why shouldn't you be allowed to play 4v4 on Brothers or Mandala?
And, still, what about Exile? I think that is a good, innovative, map.