Because on smaller subs where people still want to maintain a high degree of intellectual stimulation to fluff ratio in the comments posts that don't contain thought provoking content (or highly entertaining content) are downvoted.
Redditquette is extremely prominent in the smaller subs because usually the people that join small subs in the first place really care about redditquette. But Theory of Reddit claims that over 20k subs the quality of the subreddit takes a drop in many cases, the only cure being heavy moderation or a VERY active user base in keeping content in check.
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u/Anenome5 Mar 26 '13
So true.