Ask HN: Why 30 posts per page?

I'm sitting at my desk eating lunch and browsing HN and a thought occurred to me--why does HN display 30 posts per page? Is there some attribute to the design that makes 30 an ideal number for usability, or some optimization on the backend that makes 30 the ideal number? Why not 25, or 35, why not infinite scroll? Is it because "it's the way it's always been, and users are used to it?"

I am not a technical person but enjoy the content posted here and love learning about why things are the way they are. If anyone has any background on this from either inside YC or out, I'd love to hear it.