Answer: Because if human behavior and beliefs were actually influenced factors other than pure reason, then his belief in Atheism could be seen to be just as much a product of those non-cognitive factors as those of Theists. So, then how then could Atheism be superior?
There is an anthropological contradiction here.
Dawkins believes that humans are the products of evolution- so humans are merely the products of their genes. If that's so, then they would develop beliefs that were useful and not necessarily true (this is Alvin Plantinga's argument). Some beliefs could be true and useful, but it's hard to tell the difference, since there is no knowledge of reality that isn't mediated by the organs that our genes have given us. If that's true, Dawkin's Atheism is no more rationally justifiable than Theism. Atheism and evolution itself are merely useful beliefs, we have no way of telling if they are actually true and inherently rational. For this reason, he cannot claim himself to be superior to Theists.