Air Travel covers much greater distances much quicker.
Pilots are much more willing to transport a dog 600+ miles where as a ground transport is going to be much more limited in range.
Pilots love flying, any reason for us to fly is a good reason.
Dogs are more comfortable with quicker travel times. We want to reduce the time en-route which should reduce overall anxiety and stress in the dogs. All the dogs we have had the pleasure of flying have reacted well to being a plane. Most of them fall asleep, almost as if they know they are headed towards a better life.
Also Air travel involves fewer people to transport. a journey that needs 4 plane legs could have needed 10 ground legs.
There are times when ground transport is more effective, such as when there are long periods of bad weather, or the transport consists of more dogs than we can fit on a plane.