Is it because Luffy is the captain? I think people need to look at details. Luffy's weakness is piercing attacks , which is Zoro's specialty , not to mention how powerful he is as a swordman , those facts speaks for itself. Secondly when Kuma extracted Luffys damage he had taken in the fight with Moria , Zoro took it and was still standing , where as Luffy was unconscious after taken that pain. And I'm not just saying this because of these facts , I think Zoro is genuinely stronger.