Why? He clearly feels pretty strongly about his beliefs. This feeling does not seem to spring from personal bad experiences, yet there does seem to be something personal in his advocacy of what I might call materialism.
Everyone seems to have a "threshold of the absurd" holding that things beyond a certain intellectual borderline just cannot be true. This seems to be personally based and to have nothing to do with external facts. The borderline beyond which individual belief cannot be sustained varies from person to person. One person may harbor the belief that ghosts may exist, while another will hold they are an absolute impossibility because they are beyond his belief threshold.
What determines our threshold of the absurd? It seems for many to be a feeling from within. What provides this feeling?
While I do not profess to have the complete answer, the temporal lobes of the brain seem to be involved here. These lobes seem to incline one to religious belief or beliefs in things beyond the current paradigm, from UFOs to mermaids. The size of your temporal lobes seems to bear some relation to the number of things you find it possible to believe in. In extreme cases the lobes can cause hallucinations, caused by temporal lobe epilepsy or temporal lobe lability; but many people can have wide belief systems beyond the realm of the material, perhaps totally engendered by these lobes, without experiencing either of those two conditions.
Now, it has been established that Richard Dawkins has small temporal lobes. I suspect that their smallness causes his disbelief in things that are beyond current scientific analysis. He does not find the paranormal credible because he lacks the brain stimulus to do so.
I understand he is aware that the majority of people have larger temporal lobes than he and are therefore more prone to a wider spectrum of belief than he entertains. As he regards the non-material as absurd, he thinks the human race has a built-in delusion factor and that a human race that generally thinks as he does is never likely to appear. This he finds depressing. Hence the vigor of his crusade (if he'll excuse the use of a word with religious tinges) against beliefs that fail to fit his concept of reality and his disgust at negative elements which arise from such beliefs (as they undoubtedly sometimes do) and his lack of appreciation of the positive ones.