One has to keep the target audience in mind.
1) There's the previous point showing how big Christianity and Islam are in the world.
2) Dawkins wasn't aiming for the 2.5 billion population Indian and Chinese market nearly as much as North America and Europe. He's speaking to them, primarily. It would be a worse making approach to try to appeal to every person equally, including addressing every argument or point in the long tail, and including addressing equally the parts of the world that aren't going to read your book nearly as much with some big gray vague statement, than to target a big but focused audience and nail it decisively with a big bold title and sell millions of books.
1) There's the previous point showing how big Christianity and Islam are in the world.
2) Dawkins wasn't aiming for the 2.5 billion population Indian and Chinese market nearly as much as North America and Europe. He's speaking to them, primarily. It would be a worse making approach to try to appeal to every person equally, including addressing every argument or point in the long tail, and including addressing equally the parts of the world that aren't going to read your book nearly as much with some big gray vague statement, than to target a big but focused audience and nail it decisively with a big bold title and sell millions of books.