People like edgy serious characters, simple as that. But edgy isnt the issue, its that zoro is one of the worst written long running manga edy characters.
Almost 1100 chapters and zoros depth never reached the likes of Vegeta, Sasuke, Killua, Scar/Mustang, Hiei. Hell, even bakugo has more to offer.
In the past 500 chaptera WTF has zoro done thats deeply thematic, poetic, contemplative, philosophical, any fucking thing other than "i will get stronger to beat my opponent and look cool while doing it".
Oda fumbled with yasue, because it seemed like he and zoro were having a moment up until his death.