K. Rool is not comparable. Sakurai has never commented on K. Rool as a fighter, pretty much the main reason K. Rool wasn't added before now is because Nintendo didn't actually believe the character had appeal enough to keep him around. They essentially assumed he was non-integral. Him being in Smash is a much more obvious case of "we just didn't think of adding him up until we saw the fans asked for it", and the inclusion was justified since he still has a pivotal role as DK's most iconic antagonist.
Geno is different. He's a one-time side character that hasn't showed up in anything of note for decades. Sakurai has admitted that he wanted Geno and that he realizes the character was popular - all the way back since Brawl in fact. He wouldn't need a ballot to determine Geno's popularity. Why would Sakurai provide a full disclosure on the character if he still had it in him to want to make him a player character? Probably because he doesn't think the character is all that worth fighting for, or because he might've changed his mind on him. Hell he basically admitted that a Mii Costume is perfect for him since the Mii Gunner has an arm cannon.
Plus, Geno is the only Mii Costume to have gotten a fighter splash card, as if to say "he might not be a fighter, but let's give him something in spirit of being such". Any character that's been added so far in Smash Ultimate has, even in spite of the ballot, been mostly informed by staying notably relevant or having a sort of iconic status to them - which Geno is neither.
