Darksun races differ from the races from other worlds, a lot of mutation, ravages of magic, and survival of the strongest

Half-orc, Gnome, Dragonborn and Tiefling do not exist on Athas. Orcs and Gnomes have been extinct for a long time. as far as most people know there is only one dragon in the world. and since Athas is almost blocked off from the other planes of existence not enough Fiends have visited to create a viable race of Tieflings.