Well that’s a good point and something I’ve wondered about too. Over at WEIT, Jerry asks why the faithful don’t debate each other. Many religious opinions are fundamentally incompatible, even within what is nominally the same religion. The most obvious examples are to do with whether the Bible should be taken literally or which bits should and shouldn’t be considered metaphor. You’d think the proponents of these ideas would spend a happy lifetime arguing with each other about who’s right. But they don’t. Or they don’t seem to. If anything, they ignore their differences to unite against the common enemy of reason.
I think Jerry’s answer is the right one:
Because, I think, religious people realize that by attacking someone else’s superstition, they undermine their own. By exposing the lack of evidence for the other guy’s faith, you inadvertently expose the lack of evidence for your own.
When faith-heads debate atheists they can and always do play the faith card. They know they are right not because of evidence (indeed, to them evidence is insufficient) but because of faith, which they think is good enough. But if they debate each other they both have to use this trick and it doesn’t work.
Not that it works against atheists either, but when its faith against reason, the faith-head can pretend she has the authority of all the religious behind them. If it’s faith against faith, it just looks like your-word-against-mine and quibbling over the details.