I think the better question is, why are any mounts ground only?
This is a world of magic and fantastic technology, there is no real reason that every mount in the game shouldn’t be able to fly.
Make it an unlocked buff like the Siren Isle buff, only it puts a cloud under any mount that is ground only and alows them to fly anywhere flight is enabled. It could be from professions like an Engineering contraption like a Solid Cloud Generator, or a magical effect from Enchanting or Inscription.