Because there's a high variance on your estimate, so in a lot of circumstances just by variance you'll show users bad comments on top most of the time, rather t

Because there's a high variance on your estimate, so in a lot of circumstances just by variance you'll show users bad comments on top most of the time, rather than showing them the good comment that you are certain of. In this context, the pessimistic prior is more of a hack than the confidence interval, because your mental model isn't that most comments suck. Your mental model is that most of them are ok. The pessimistic prior is just there to say, if 1000 people confirm that a comment is ok, we should show that over a comment for which we don't know whether it's ok or not yet, and this reasoning is much better modeled by a lower bound.