This piece about a prominent secularist's new book helps explain why. Austin Dacey is the author of "The Secular Conscience," where he argues that secularists have sought to preclude religious and moral claims from public conversation, through the following reasoning:
[S]ecular liberalism has come to hold that because conscience is private or personal, its moral conclusions must be subjective, and because conscience should be free from coercion, its moral conclusions must also be free from public criticism.
He argues that in doing so, secularists have made a terrible mistake. Sounds like an interesting book, and a valuable one.