Luffy and Zoro have not been given the same level of stakes with direct confrontation against Kaido.
Luffy is the one who set up the alliance both in Punk Hazard and in Zou, Luffy is the one who said he'd beat Kaido 400 chapters ago, Luffy is the one who took down Joker, Luffy is the one who Kinemon, Luffy is the one who had a 1 vs1 matchup with Kaido and lost, Luffy is the one who realised Kaido's specific weakness rather than rely on a tangential method to bypass the scales, Luffy is the most "Oden like" both in terms of portrayal and according to Yamato. Ultimately Luffy is the one who will become pirate King, and that is in direct opposition with Kaido and the Yonko, to the point that Kaido demanded Luffy dare to claim that again in front of him.
Zoro has some moments sure, but well well below the sum total of Luffy's moments.
Law is the one who proposed this alliance before the samurai were involved, and he likes Kinemon too in his tsundere way, but yeah he is last.