One reason HN shouldn't use infinite scroll that I'm surprised no one has mentioned yet - this site doesn't have a high enough rate of new content for it to be worthwhile.
Ironically for a SV based forum full of web developers and startup enthusiasts, Hacker News can be deeply contrarian and antimodern as a culture, so not updating the UI (or at best, only updating it very rarely) has become a matter of pride, as well as a lifestyle brand for YCombinator.
Personally, I'd like to see them add it just to watch HN lose their collective minds.