I'd say that it's because people with religious beliefs don't want to show it online. Whenever I see anyone "own up" to being theistic, a bunch of angry pseudointellectuals descend upon them like a pack of rabid dogs.
By and large, I find the way that the more "radical" atheists on the 'net do this hypocritical - lots of them use the excuse that religious groups persecute the non-religious, but what does that make them when they spend their time persecuting people who have done nothing to them? The word "confrontational" doesn't even begin to describe it. They seem to adhere to the strict philosophy that, yes, people should express their beliefs on the internet but if they believe anything other than nothing, they're wrong.
Also, for one reason or another, being atheist has become the "intelligent" thing to do - most teenagers these days won't even consider the idea of a deity, because it's become the dumb thing to do. I partially blame hipsters for this attitude. Yes, religion has in the past inspired a lot of hatred and blind belief, but we're not in the past any more. Those who are modern human beings and want to partake in a religion should be free to do so without any fear of having the blame of the past put on them.
Most people who are religious won't say anything about it except to their closest friends any more. This is, in part, due to the fact that I've found in the modern dichotomy of the religious and the atheist, the atheist is more likely to be the one who refuses to keep their belief (or lack thereof) to themselves - if you've ever met a teenage "radical" atheist, you'll probably know that they never, ever shut up about religion, even among friends with whom the topic is tired and stale. When you combine that with the anonymity of the internet, you end up with somebody whose ability to shout at others about faith outmatches even some of the most irritating zealots.