[rweba]

I think there are a few factors:

(1) The culture started by Paul Graham, back when he used to write lots of essays, emphasized a certain "hyperrationality" and attracted a lot of super-logical people in the beginning.

(2) Heavy moderation to keep things from deteriorating and discourage overly polemical debates.

The bottom line is that people quickly get an idea of what is tolerated and what is not tolerated so they work to stay in line which doesn't seem to happen as much on other websites.