I refer to this one as DarkShikari's law: "Any community that gets its laughs by pretending to be idiots will eventually be flooded by actual idiots who mistakenly believe that they're in good company."

https://news.ycombinator.com/item?id=1012082

It explains the death of so many communities, and also sorta works for 4chan, despite the perpetual state of undeath that 4chan is in.