User blog comment:Staw-Hat Luffy/Why Kaido?/@comment-972068-20130204174116

"The order doesn't really matter." ~Monkey D. Luffy

He never said he doesn't want Shanks first.

Maybe Law is aiming for Kaido first because he's the strongset, so it's good to get the strong one out of the way. Or maybe he likes the challenge. Or maybe he has a personal grudge against him. Or maybe because Oda forgot that it was X Drake who went to his island, and accidentally put Law in the story and it was too late to switch back.