Well, here's a thought, or two.
It may not be clear to folks who happen to land on the site that they can see all there is to see without being a member. So they sign up, thinking that membership will 1) provide access to more 'stuff' or 2) make finding information easier or more complete.
I also think that people who 'find' the forums are doing more looking for information than they are trying to share information.
Just thinking though.