I can't speak for anyone else, but in my case it's usually a case of a) genuinely (if usually foolishly) believing I will get back to it someday and b) leaving the sites up as a resource, even if they are never updated, becasue people still have them bookmarked and may find them useful. And people are always discovered fandoms at their own rate, regardless of how long ago thsoe fandoms were new and shiny. They're always going to be new and shiny for someone, somewhere...