I hate it, and I hated that WCI wasn't sanji's arc either. Wanking these stupid side characters I couldn't give 2 shits about doesnt matter at all to me.
Whole Cake Island wasn't about Sanji
as a fighter, but the plot of the arc clearly revolved around him.
To begin with, it was only because of Sanji that Luffy went there, the main event of the arc was Sanji's wedding with Pudding, Sanji's whole family appeared in the story, we got a lot of details about his childhood with the flashbacks, and he faced very significant situations in which new layers of his personality were exhibited.
Zoro, on the other hand, did shine as a fighter in Wano, but the plot of the arc itself did not revolve around him; He was just a fighter in the Alliance among others, and he only had a very small flashback without much relevant information.
That's not to say he didn't have his moments that weren't related to fighting; His interactions with Hiyori, that laughing man who died, and that girl who began to laugh during the execution making Zoro angry were certainly something. But none of it was significant enough to show new layers of Zoro's personality; He is still the stoic Buddhist monk who is loyal to Luffy and has a rivalry with Sanji.
He got stronger with Enma, and unlocked Conqueror Haki, yes, but these are just power-ups, among many others he had throughout the series.
So his change in the arc has been just a matter of extent, and not of substance.
In terms of character development, Zoro's power-up was nothing compared to Sanji having a internal struggle considering whether or not it would be worth it becoming stronger with the cost of losing part of his emotions, which ended with him breaking what had been his last power-up, and being forced to accept the change made in his body precisely by the technology made by his father who he hates.