Wano has least sales compared to every other arcs. According to you, sales should pump up as Zoro got heavy focus, infact even more than Luffy. And now all SH getting focus, it will jump to 4 million again right.
Zoro had terrible plot line with Shusui and getting Enma, that was one of the main buildups that was destroyed, but again sales are down because of my below comment and Oda again casting a Wide net while still trying to retain focus on Luffy
When Oda had time to build up plot in wano with Robin, Usopp, and Franky, Im taking Zoro out of the equation here. The 2nd chapter in he has Luffy come in, meet someone get over emotion and destroy staying low key in Wano. Again Same pig different lipstick
Oda spent years building up Wano to build up Wano even more in Wano- this is why sales are down