@JakkRo Welll, that mix plus god-tier writing and top notch art are the secrets to its success.
I often don't really think about the art in this while I'm reading it because I'm not that art-oriented as manga readers go . . . but if you look at the art in this and then go look at a typical manga, whether shoujo, shounen, seinen or josei, and think about it for a moment, you'll start noticing things like just how much more same-facey most manga are. In this not only does everyone have a quite individual look, they also have individual expressions--the harassed look of one character is noticeably different from the annoyed look of another or the seething look of a third. The art supports the characterization. It also means that when a minor character shows up you instantly remember who it was and don't get pulled out of immersion while you try to figure it out. And there are so many other things about the art--the liveliness, some of the surreal scenes like the deep sea shogi metaphors and so forth . . . it's seriously good.