The thing is, the "more" links are specific to each user and generated page. They're not just "page 3 at the time you click on the link", but the actual next 30 links following the 30 it's previously shown you. As a result, it needs to store state for each of these links it's created (IIRC, the fnid is a reference to the closure containing that state). For obvious memory management purposes, these need to be expired after some time.

I agree that it's an odd implementation choice though. I guess it made sense when HN was smaller and it could grant more ressources to each user. Nowadays, Reddit's simpler system (with risks of duplicates or missing rising links as you go deeper) would probably make more sense.