I suspect it's at least partly to do with the subject matter. If the community is united around a common interest, they can (sometimes) put aside their differe

I suspect it's at least partly to do with the subject matter. If the community is united around a common interest, they can (sometimes) put aside their differences elsewhere to talk about that one thing in a moderately civil way. If it's a free for all about anything and everything, then well, it's like herding cats and a vicious flame war is likely to ensue at some point.