If you’re looking for an in game reason it wouldn’t be too difficult for Blizz to pull something out of their butt. Not having flying doesn’t really have to do with any story reason
World of Warcraft is full of memorable moments that are only possible when players explore the world by ground.
However, while we firmly believe that keeping your feet on the ground is a key part of discovery in WoW
Blizz thinks that everyone should be on the ground to form some memories 