Because killing Kaido is framed as a challenge, the same way beating him in a 1-on-1 is. So, naturally, you want one of the main characters to overcome one of those challenges.
Also, Zoro’s path to WSS, especially post timeskip, has been very lacklustre. Giving him a good Wano-arc, where he does something very significant, makes it so that we don’t get rushed and random Mihawk hype at the end of the story just to validate his “WSS” title, lol. You have to build that credibility overtime, and landing the killing blow on Kaido is the start of that.