Because, whether people want to admit it or not, Zoro is the most engaging character in One Piece. :cheers:
Unlike Luffy, who is bolstered by destiny itself to whatever path he chose, The Grandmaster created his own path. Which means, in his journey, there will always be uncertainty. And it is this uncertainty that draws the attention of people, because, while uncertainty is scary, it is also very thrilling. That's why people who hate him cling desperately to see him fail and that's why people who champion him cannot wait for him to triumph.