I thought your question was interesting as I have a small christian forum, and the same thing happens there. I actually thought it was because it was religious and after all... who wants to be bothered with religion.
But then I figured the percentage of active users to # of members on both this forum and my forum and voila! They are identical! The only difference is that my membership is tiny.
Soooooooo, in summation, I'm guessing that is the nature of the beast (forum)! People would rather lurk than post, and that's O.K. by me.