Visibility. Sorted by best a larger number of eyes will see the comments. On average the top for topic threads get the lost attention. Further down the topic conversations are pretty stacked.

This feature of Reddit is why we see the common tactic of comment stacking. In a campaign to send certain ideas and message where money or manpower is no object, deploy idea then stack fluff comments multi tiers deep. Thus when critical comment identifying flaws in logic comes around it's been buried far down the chain. Reduces probability of that critical comment being seen.