It's possible the title Avatar: The Last Airbender could have been a misnomer. The series begins with Avatar Aang emerging from a sphere of ice and being told he's the last airbender. Throughout the series, Team Avatar journeyed to various places including all of the air temples to confirm the statement. It would seem that the airbenders, as history tells it, were victims of genocide perpetrated by the Fire Nation in search of the next Avatar.