I think perhaps it's the size of the textbox and the lack of limits. I super briefly touched on it in the context of Dreamwidth after hearing from several friends who find posting here too intimidating (like a big textbox requires big thoughts), compared to Twitter which encourages you to post short thoughts, or even Discord showing you how to type just one line by default. That means that just saying "I enjoyed this" or a sentence or two feels like it's not enough in the context of an AO3 comment box and you should say "more" which is effort which you might do later which whoops closed the tab XD I think people find it easier to "react" to a story on Twitter and like "just gushing" is more appropriate in that context rather than on AO3 itself (they're wrong, but from reading between the lines and looking at the culture, that's my understanding/feeling.)

Incidentally, that's the same kind of thinking I heard from some AO3 authors I used to enjoy who moved to posting fic exclusively on Twitter. In addition to the ability to "like" specific sentences, they say it feels like less pressure posting there, the work doesn't need to be as polished -- even though AO3 has no such requirements either, and their fics are excellent either way, I guess AO3 gives off this serious impression.