Because Christianity is closer to home, dear, it's that simple. It's always been atheists vs. theists, Islam has only cropped up recently because of the huge wave of Islamophobia the media has directed at it. Both America and Britain, though, are very Christianity v. Atheism, because we learn more about Christianity through the abundance of churches, and all the atheists who have been dragged there as a child will know a lot more about Christianity because of it. Mosques aren't as common.
Get on their case if you want, but I won't be that interested.