It might be related to Luffy's potential, as Kaido saw just how strong Luffy became in just two short weeks. Characters like Shanks, Big Mom, Mihawk, Admirals etc... are already at their peaks. Considering Kaido was just confirmed as the strongest, those who already exist at that level wouldn't excite him, because he knows where he stands in comparison to them. But seeing Luffy go from where he was to where he is at now, in such a short amount of time, has piqued his excitement. At least, that's one potential reason for it.