I think it's because of the nature of the site. It's part of the same reason folks don't use their real names. This is not a dating site where names and pictures are relevant to the conversation so, most people just skip it. I had a couple of pictures up here.. but I decided I preferred being a bit more abstract than that. Looks can be distracting.. I'd rather focus on the words.. (and the voice if I could hear it.)
I prefer Avatars and Screen-names to Photos and Real Names.. there's more of freedom of expression there. And they reveal just as much, if not more, about a person than their photograph and Real Name. What we name ourselves and how we choose to appear says a whole helluvalot about who we are.
juxtapose that with Facebook.. or even myspace.. most people have a photo of themselves there because that's what those sites are about.