but why do readers find it hard to comment on a novel chapter but easier on a manga chapter
Probably because your comment isn't likely to be seen by the creator if you comment on a manga chapter. You comment so that other fans can see and interact with you. Even on translated novels, you only expect to be seen by fans and the translator - who's technically a fan of the series too.
You get self-conscious if the person who made the thing is watching, though, because they take everything personally. Especially on Scribble Hub, where comments on a story are directed mostly towards the author rather than other fans, so everything you might usually say takes on a completely different meaning.
On a manga chapter, you could rant about how much you hate a particular character or don't like a certain development in the story. On Scribble Hub, that would be an attack on the author.
For manga, you can speculate with other fans about what's going to happen next and talk about the things you really hope don't happen. Here, that would sound like you were demanding that the author write the story the way you want them to.
You're basically stuck only ever saying 100% positive and simple things about the novel, or taking the risk to provide suggestions/feedback - which the author might get huffy about. Who can really be bothered commenting in those conditions?
You, as an author, can accidentally dissuade people from commenting too. Because you can see your readers' comments and your readers know that, you're really expected to interact with those comments.
Someone who leaves a comment - particularly a longer and well-crafted comment - really expects some sort of reply. If the author doesn't reply, they'll feel snubbed and you'll come across as someone who's cold and doesn't care about their readers. Since they feel like you'll just ignore whatever they say, they give up on writing comments. It can sour how they feel about the novel overall as well.
So if you're getting less and less comments as your novel goes along, it might be worth considering if it's your PR that's failing you.
Those are just my observations.