well I mean if you look at it, its the form, most commonly used by Dragons them self, Look at the Aspects
Alex = Elf
Ysera = Elf
Malygos= Elf
Nozdamu = elf
Deathwring =Human
Kalegos = Human
Its only a hand full that pick different like Chromie and Ebonhorn. While yes id love for it to be all the races, of Azeroth it just boils down to lore wise, its what msot dragons go with anyway