People come to HN comments to have a constructive debate, so they are less likely to downvote for disagreement. I often upvote comments I disagree with if the point is well made.

People come to Reddit to have their beliefs confirmed. The subreddit structure reduces mixing between users with different beliefs, and where that mixing does occur, e.g. on the large subreddits, it tends to turn into a downvote battle.

^ It's not really about intelligence but openness to new ideas. You can have one without the other.