A few years ago it was decided that the traffic of the forum is too large to keep the forum open, so they closed it and only fan club members can access it. There's also the "online club" option which is 10 british pounds a year, and it basically gives you admission to the forum.
In my opinion, t could have been kept open to all visitors, but with a quota for non-members. The forum at thefuselage.com (official Lost fansite) has a system that prevents unregistered people to access the forums when the traffic is at peak levels (usually the 2-3 days after the current episode has aired).