It is stated the air bending population was the lowest of eny nation. I think the reason why is the air nomads were nomads and moved around alot so they would often settle down in other nations. The airbending allele is recessive outside of their own people, this is because air is the most spiritual element and the entrance to the spirit world was closed. This is why the airbenders went extinct because those who escaped the genocide were forced to blend in and well they settled down and their children did not inherit the trait. This did not apply to the avatar's allele because the avatar is linked to the spirit world, so him and his blood line was fine. When the spirit portal was oppen it gave carriers of the allele air bending abillities.
Most air benders did not settle in the water tribes due to the cold which is the reason not many Water Tribesmen are New Air Benders. Most airbenders are in the Hu Xin provincess due to it's closness to two of the most populated tempels so alot of airbenders fleed to their after they escaped the genocide. The racial supority going on in the fire nation could cause those of air nomad history to be sent to the colonies. After the genocide many airbenders would want to hide in the walled cities of Ba sing se which is why so many new air benders were in Ba sing se.
What do you guys think?