I would also like to add a personal thought, cause I've been promoted several times as well and like you I have been puzzled by my popularity rank (behind way less popular series).
I have 14k subs but I struggle getting more than 300 likes per update.
I see series with half subs getting double the likes.
I made some experiments and if I make an episode designed to push people to like and write comments they do it, otherwise they forget or don't bother.
My conclusion is that (also taking into account everything that has been said so far, that it accurate) also your viewership matter. To put it simply: if your audience is really young you might have more chances to get more comments and likes.
If you're comic doesn't cover controversial topics, doesn't ask for readers' advice directly or indirectly and appeal to an older audience, it simply won't automatically get as mush engagement.
As a creator and someone who knows the struggles of creating engagement, I myself forget to put a like or a comment even when it is totally worthy it. It's just because I'm in my 30s and I don't spend much time on social media and this is just not natural to me -_-
I hope this help. Cheers!