Because Zoro has spent all of Wano until the raid wandering and growing accustomed to the land and its people. He's experienced first hand how bad Kaido and Orochi laid waste to the country, from innocents being cut down by maniacs that make up the regime, starving children and elderly from the poor districts, forming bonds with the various individuals, such as Yasu and some of the Scabbards, and knowing that Kaido is the source that allows the misfortune of the country to persist.
Another reason could be on a fundamentally spiritual level. Zoro probably holds a greater attachment for Wano because he might, subconsciously, recognize it as his homeland.
Lastly, Zoro's fundamentally a good man. He's met several Wano citizens who've shown him kindness, and so he feels compelled to repay the debt. It goes back to him helping Rika way back in the beginning.
Addendum: It's not like Zoro didn't want to fight Kaido before Wano. He was already resigned to fight Kaido and his forces from the start. It's just all he's seen during his time on Wano have strengthen that resolve and fostered a desire to take out the main man head on.
Another reason could be on a fundamentally spiritual level. Zoro probably holds a greater attachment for Wano because he might, subconsciously, recognize it as his homeland.
Lastly, Zoro's fundamentally a good man. He's met several Wano citizens who've shown him kindness, and so he feels compelled to repay the debt. It goes back to him helping Rika way back in the beginning.
Addendum: It's not like Zoro didn't want to fight Kaido before Wano. He was already resigned to fight Kaido and his forces from the start. It's just all he's seen during his time on Wano have strengthen that resolve and fostered a desire to take out the main man head on.
