Seems like a natural progression of a person's interests. When I started reading HN in ~2009 almost everything on the front page was new to me and I spent a ton of time reading articles and comments. Now, I've had almost 15 years of working and reading and studying and the only things that I don't have at least some passing familiarity with are extremely technical or news or political things that largely don't interest me. I spend a lot less time on HN now than I used to and I don't think it's because the site got worse, just because my interests have moved past what the site offers.