Because he is a representative of science, and a very vocal, well-published one. He is a highly credentialed biologist (evolutionary biology, genetics) working at one of the greatest institutions of higher learning in the world (Harvard). He is not some obscure "nut case" auntie in the attack....
Rather, he is highly esteemed by the scientific community, and is a man of great influence within it.
Did I answer your [implied] question???
He is just one of many. Why consistently choose him? He is well published, so there are many quotes to choose from. Why is that specific quote repeated over and over?