My guess would be that there's a feedback effect, where posters learn to only post comments which are likely to get upvotes. The net effect is that shared opinions converge. Ideally a comment section would be a sample of the whole distribution of opinions, while in practice only the mode of this distribution is represented. This is a direct effect of having votes on comments.

With regards to your question, what I meant was that these two aspects are in fact non-separable. The technical solution (upvotes) shapes the social behavior (the type of discussion) very strongly.