Well, lots of porn / porn-ish stuff has ecpms well below $1. I'd bet that includes /b/, the image board, the rapidshare links, the torrent board, etc. Further, I can't imagine many ad companies are jumping up and down to serve ads on boards associated with that kind of stuff -- viz fark having to separate the main page and the boobs page.
Even worse, most advertisers don't like forums. Add the two together and well... bummer. Hard to make a living like that.