I'd imagine it's just because earlier comments have more time to gain points than later ones, and as a comment gains points it also gains visibility, which kind

I'd imagine it's just because earlier comments have more time to gain points than later ones, and as a comment gains points it also gains visibility, which kind of snowballs. If you think about it, it wouldn't really make sense to have comment rankings decay. Breaking news is only breaking news for so long, so it works for the front page, but a valid, insightful comment on a particular story doesn't lose value over time. Side note... I might be wrong, but I think the stories use a rational decay (current point value divided by time since post) and not an exponential one.