Moderation and composition contribute a lot to a sub's quality. On one end you have fantastic and popular subreddits like Ask(Historians|Science|*) which rely on extremely heavy moderation and qualifications or sources. On the other end you have very niche subs that are really only of interest to enthusiasts/relevant parties, and so composition keeps the quality high rather than moderation. The problem with a lot of subs is that they are popular and unmoderated, and so left alone they just converge to the mean (and the mean's not great).