Bulliet has an entire book ("The Camel and the Wheel") in which he takes great care to address this point. As another reader has pointed out, harnessing camels

Bulliet has an entire book ("The Camel and the Wheel") in which he takes great care to address this point. As another reader has pointed out, harnessing camels is much more difficult than harnessing a horse, and camels are not good draught animals. Bulliet recently published his history of the Wheel, which is excellent ("The Wheel: Inventions and Reinventions"). I highly recommend it. I'll also add that Bulliet's "Hunters, Herders, & Hamburgers" – though unfortunately titled – is one of the best books around on the history of animal domestication.