My private hypothesis here is that we're doing a better job as a society of encouraging "disconnection" from being "always online," which is documented to aggravate mental health issues[0].

It stands to reason therefore that the people who remain online to comment may have lower levels of mental hygiene by virtue of their ongoing exposure to the internet and social media, thus resulting in a gradual decrease in sentiment over time.

[0]: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7364393/#:~:tex...