The technical reason is that stories get automatically unpublished if they have more than 10 ratings and fall below a certain ratings level (I think it's 2.5). A lot of the really random stories still up are from the early 2000s and 2010s, when people were a lot more generous with their ratings. If they got published today, many of them wouldn't survive current standards. Also, there's no quality control on unpublished stories, which anyone can read if they're made public.