Zoro is popular because of Oda, not because of anyone else. If Oda decides to use him more and if he decides to use him differently he wouldn’t be this popular, because Zoro is not an effective leader like Luffy nor as powerful as him, he doesn't have strong ideals about the pirate world and life, he doesn't want to change the way of things like Luffy wants, and you can't give him more panel time than he already got.
For example, when Nami leaves, Zoro says forget about her, but Luffy says no so they went after Nami to save her from Arlong. When Sanji had trouble with Vinsmokes, Zoro says forget about it, Luffy says no and goes to fighting with Yonko for Sanji. That's the difference between the two characters. You can't turn Zoro into an effective leader like Luffy. So he will stay as he is, like Sanji.
Oda would never do that, because that means we will see more of Buggy vs Zoro, Arlong vs Zoro, Enel vs Zoro or Lucci vs Zoro, or Oars vs Zoro type of fights, and he will turn into a complete loser who loses to every opponent that Luffy beats, Oda doesn't want to kick Zoro's ass too much in every single arc, that's why he doesn’t get more screen time, because he won't be cool if they kick Zoro's ass in every single arc, I am sure Oda is thinking very hard on what will he do with Zoro without humiliating him against the 2 Yonko right now. He kept Zoro away from Doflamingo, instead he humiliated Law and Sanji, but there is no escape now, Zoro will get his share of beating again.
Turning Zoro to a garbage who gets beat everytime wouldn't do any good, you might be fanboy enough to believe Zoro can do the things that Luffy can do but he can't, so that’s why he doesn't get more screen time, that’s why he doesn't have his own crew like Law and Kidd have, that's why he is Luffy’s underling. He can't be as important as Luffy is. Either separate Zoro from SH crew and turn him into a rival captain to Luffy, or he will stay as sidekick and not humiliate him against stronger opponents that Luffy beats, so it means he can stay cool and popular for some fans. Zoro being popular is directly related to the current screen time and portrayal he got from Oda, Oda is a smart guy and he knows how this things works.