My guess would be because of how WHM is a healer class, Holy is a risk due to the high MP cost + WHM's lack of mp recovery for extreme AOE dmg. It's very easy to stomp your mp to nothing, and if you don't have Shroud or emergency ethers available, you're outta luck if the tank needs above average heals after words!
WHM still gonna kick almost everyone's ass at AOE trash anyway because Aero III is actually pretty baller + Assize, so regardless!