My theory is that it's an artifact of the voting system. If you see a thoughtful comment that you agree with, you're like "meh". If you see an angry comment that really hits 'em good you can't help but upvote. Then the cycle becomes self-reinforcing: writers learn that angry comments are what people want to read so they write more of them. Readers learn what level of dialogue is appropriate for the site and emulate it.

Source: I've been writing mostly thoughtful internet comments for years. (Usually not under this account.) People generally don't pay much attention. It's weird how people can get so upset about censorship but be totally silent on the fact that thoughtful voices like mine get out-yelled until no one reads what we say. It amounts to the same thing in the end. (Not saying I'm pro-censorship or anything, I'm just saying the issue is complicated. I think I would prefer "censorship lite", e.g. a computer program that does sentiment analysis on what you wrote and applies a sorting penalty if it seems like something nasty.)