There are a few small reasons why.
Though part flying, Gyrados does not resemble much of a flying type, more of a water/dragon. Yes, other non-flying types have Aerial Ace, but others that do have access to that move typically have something sharp to cut or to slice. Even the sound from the attack suggests some sort of slice or cut, and I don't see an acceptable way Gyrados slices something. Maybe with its teeth, but that would count as a bite, or a fang move. The tail may seem like a possible canidate to use Aerial Ace, but the tail doesn't suggest a sharp blade or possible cutting move, based on design.
Is that an acceptable answer?