why is Zoro more popular than Sanji?

  • Sanji got two significant backstories compared to Zoro's one (very brief)
  • Sanji got his own arc, while Zoro is yet to get one (Wano is not a Zoro arc)

It seems like ODA gives Sanji more special treatment than Zoro,
yet Zoro remains the more popular character. Why is that?