I don't think people are not debating Islam because they are afraid. I think that there are two things to it. First is that Christianity is by far the dominant religion in North America, so not only is it the most pressing concern it is also a lot easier to find a Christian to debate. Throw a rock into a crowd (only if you're without sin

) and chances are you'll hit a Christian, not a Muslim.
The other thing is when we do get into it, we get a little queasy because we wind up being on the same side as the asshole Christians who start wars and want to promote all sorts of discrimination against Muslims, while most atheists are the first to oppose discrimination based on religion. And they were the guys we were just debating yesterday!