Part of it might be due to the rise of static site generators which almost by definition do not directly support comments without having an additional non-static service, external or not.

Even just having an endpoint to submit comments seems a lot to ask when the site otherwise doesn't even need a database.

Ultimately I think the need/want for discussion is better served by different websites rather than asking every site to provide comments. It would be nice if browsers could integrate or at least link those comments for you.