Even in a zone defense you have some clue as to what's going on with a play just by looking at formation. All defenses work off of keys that the offense gives them. You automatically know no more than 6 players will have an opportunity to end up in your zone, and specifically you know which 6 players those are. On top of that, you can usually winnow it down to the 2 players that are on your side of the field. You know going in whether the offense is going to have a balanced formation or whether they're going to overload one side of the field. On top of that, after the offense sets they only get to put one player in motion, so the keys remain rather firm. With the A-11, you know that 6 will be involved in the play, but you have no clue as to which 6. Furthermore, you have no clue as to whether all 5 of them (assuming QB still remains under center) will overload one side of the field or whether they're going to spread the defense out. With the speed of today's players, even the defense needs something to key on or the zone becomes ineffective as the receivers will blow right by the secondary as they're still trying to piece together the play that is coming at them.
Granted, a standard offense will sometimes overload one side of the field as well, but at least the defense has 5-10 seconds to adjust and react before the ball is snapped. With the A-11, the defense doesn't know the formation until 1-2 seconds before the snap - impossible to adjust to.