Because Reddit is a moving target. At any given time HN is of a similar quality to Reddit-as-of-three-years-ago and declining towards Reddit-as-of-now.
I’ve been on HN for many years and don’t notice this decline. Could it be that those who observe a decline in quality simply have gotten more sensitive to low quality posts with age?
Reddit has always been a few steps ahead of HN in its slide toward redditism, but I agree with GP that it feels like there is more disagreement-downvoting and replies that add nothing to the conversation. Not to the same degree as Reddit, but I think on both sites we’re just witnessing the eternal September as they become less self-selective niche sites and more like mainstream social media.
You joined recently, not sure how you've got any ability to say it's gotten worse 'recently.' Unless you are referring to the past 10 months or so where there was one or two things that might have impacted the discourse a bit.
HN still is the place it's always been. You have more data now, perhaps, but fundamentally it hasn't really changed. People continue to upvote and downvote in roughly the same ways, and they are used to agree and disagree just as they always have been.
https://news.ycombinator.com/item?id=926703
https://news.ycombinator.com/item?id=289254
https://news.ycombinator.com/item?id=66057
(I didn't find these, they're linked in the site guidelines.)