Because the older a good game gets, the better the community. It may be only five to ten players who login on IRC every few days to play a round or two every week, but that community is more close-knit than anything you will ever find online.
Definitely those of us that have played together for years and years (and years and years more in Xanthe's case) become more close knit and it does make for an overall more enjoyable playing experience when people know each other so well. That said, the likes of Avalon and the IRE games continue to grow in numbers so there clearly remains at the least a niche market for this type of thing.