All ties back to zoro. Zoro is supposed to be the wss and possibly the strongest swordsman ever in op. If he was over shanks it broke to many agendas. It broke the yonko agenda, the shanks himself agenda, sanji agenda, and luffy agenda. What alot of shanks fans want was for shanks to be peak and only luffy and maybe bb surpassing him. If zoro also passed him then you have the zoro=sanji agenda where now sanji has to surpass him. If sanji can surpass him the you have guys like kidd and law who had way more narrative back then who can surpass him. So now it went from 1 possibly 2 characters surpassing him to around 5 or 6. On top f that you also lose the luffy people that think he can beat all his crew combined and casually beat zoro. If zoro surpassed shanks then they have to say luffy could do even worse to shanks which they don't want to fanboy that hard saying luffy could neg or low diff shanks.