JFC is full of die-hard fans. I think that is kind of what separates it from other message boards, specifically other Jazz message boards (RealGM's Jazz board being the only other one that I know of with any kind of regular poster activity). I think that just makes it hard for new people to get into the board.
Also, I think that nearly every stone that can be turned is turned on JFC (outside of someone who really wants to/has the skill to put the effort to do long-form technical analysis), so some people may just not feel the need to vocalize opinions that are already being shouted ad nauseam. For the longest time I never posted on JFC because of this. I would just get on to read what people where thinking, but never really felt the desire to vocalize my opinion because it was generally already represented.
Also, like NAOS said, the Jazz have sucked and that probably drives down activity because some people will lose interest investing so much time into a team that isn't making the playoffs. I think you can see that in Log. The Jazz have sucked and that has brought down his activity level and he has returned with a more pessimistic outlook, probably to preserve his emotional state as getting his hopes up to only be let down isnt something he wants to go through again with this same core group of players. For the most par the positive people stick around, so Log finds the overwhelming positivity to be weird, but I think that it's just the kind of people who continue to post on a team's message board who haven't won a playoff series in 6 years (or whatever it is).