My naive guess would be it demotivates mediocre posters more strongly because they’re wrong more often.
A lot of the time, “mediocre posters” tend to be the source of the nitpicking. This is because writing up a nuanced objection takes time and effort, and requires much of the same skills as writing a good top-level post; whereas posting low-effort nitpicks is easy, especially if other people reward you with karma when you do so. (And empirically, I observed a great deal of poorly reasoned comments receiving upvotes on LW 1.0 towards the end of its lifespan, although I will decline to speculate publicly as to the cause of this.)
A lot of the time, “mediocre posters” tend to be the source of the nitpicking. This is because writing up a nuanced objection takes time and effort, and requires much of the same skills as writing a good top-level post; whereas posting low-effort nitpicks is easy, especially if other people reward you with karma when you do so. (And empirically, I observed a great deal of poorly reasoned comments receiving upvotes on LW 1.0 towards the end of its lifespan, although I will decline to speculate publicly as to the cause of this.)