The instant consensus of all the pros that Benedictus sucks is a likely contributor. People netdecking aren't going to be building it, so it has to be a rank amateur who decided it would be fun.
And that's the other problem - going Shadow Priest or Exodia Priest or whatever sounds like a fun deck to build and the decks play quite differently from one another. A Benedictus deck is literally a Control Priest with Benedictus added to it. He doesn't inspire people to build a deck around the card the same way the Quests do.