My theory is that it's two things:
1. People change. My HN account is 15 years old, and my interests and ambitions and tolerances are not the same as they were in 2009, when I was 28 years old and in a very different place in my life. When you interact with something for many many years, even if that thing stays exactly the same, you change, and think differently about it.
2. The site changes, too, of course. They aren't necessarily bad changes (and often I would say they're good changes!), but people sometimes associate change with negative feelings, especially with something they have an emotional attachment to or at least have been a part of for a long time.
Mind you, I don't think discussion quality has been declining here. In many ways I think it has been improving, or at the very least staying the same under a barrage of new users, higher scale, and low-effort LLM-generated comments.