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.

I believe it is to do with group psychology, the network effect and the drive of VC funded companies for "growth". I'm not sure about the growth thing yet though, but it kind of implies the more people the better.