It's not as though there aren't comments people make - see the numerous comments on practically any Facebook post - but that people increasingly changed their consumption mechanism, and so to a degree the response mechanism.

The consumption moved from "I'll click on my daily bookmarks to see what each of these people wrote" to "I'll follow in RSS" (RIP Google Reader) to "I'll follow their twitter/facebook/etc" and from there. And each gets you a step removed from the site and the comments...

And then the response mechanism changes; where before the only real way to respond to something written on a blog was to comment on it (or if you wanted to go longer, use ping backs from your own blog), you then got people responding by twitter replies, or indeed Facebook comments, or Medium comments/reactions, and increasingly the role of the comment on the blog itself diminished.

Spending more time in social media and less on the sites themselves is easier, lower friction, and so it becomes the default mechanism. I'm guilty of it - easier to "comment" through a quoted retweet than to go and engage directly. Double purpose is I'm "speaking" to my "audience" of the people that follow me, rather than just as a fairly anonymous voice in the crowd of comments... (but hey, I'm the type that could still drop 500 words on a chosen topic like I was in high school English class.)