IMO there's something about the style of vote-moderated public posting that leads people to want to posture as confident and authoritative, even when they don't have a lot to add. And cynicism is a cheap way of looking smart (since it undercuts the need to deal with complexity and nuance). So there's a constant bias towards posting cynicism or framing ideas cynically.

On the flip side, shorter comments are easier to read, and sarcastic/cynical retorts/summaries are more likely to get upvoted when they're shorter/funnier than effortposts. So there's also maybe a bias to upvote cynicism.