Some people may feel it's not their place to post and thus they only lurk. For example, some people may only read the boards because they have never cruised and are only very casually considering the option and looking for more information about it. So, they hardly feel qualified to post a response to a thread about cruising.
For example, I lurk LuxuryCruiseTalk. I enjoy reading the posts on there, but I myself never post. Why? Because I know I would not likely ever travel on those ships ... the cost is beyond my means. So why should I post to that board? What possible insight could I offer when I've never been on one of those ships, and likely never will be?
Blue skies ...