Pica is a villain and not even a main one at that. Also just looking at everything in totality, there's more going for Zoro being involved vs Kaido than not:
- Kaido can't be 1v1'd, it'll be a group battle
- Zoro has Enma, relationship with Oden's former sword
- Zoro wanted to go with Luffy to Kaido
- Zoro now says he wants to help Kinemon and others vs Kaido
It's now more than once he has verbally stated his intention of fighting Kaido