Has he? I mean, think about this logically: In Brawl, there was very little post-Sakurai content he could even pull from. Canvas Curse and Squeak Squad (I know what you're thinking: "Where's Amazing Mirror?" However, Sakurai did work on Mirror, even if it was in a lesser role). And SS got a full track, and partial appearance in the Boss Medley in Brawl. Also of note, he gave HAL Labs the Halberd model he made for Brawl for them to use in Super Star Ultra. For Smash 4, there was: CC, SS, SSU, Epic Yarn, Mass Attack, and Return to Dream Land. HOWEVER: Work on Smash 4 began approximately the end of 2011. Only about a month and a half to two months after Return released. For that fourth character slot, there were three logical choices: Bandana Waddle Dee, Magolor, and Galacta Knight. BWD and Magolor were unproven in popularity by that point (and hell, BWD didn't even have a name yet, he was just Waddle Dee), so I daresay at the time of the Smash 4 project plan, Galacta Knight was the most likely, and even then, he had only appeared in two games. Not a whole lot to go off of. It is unfortunate that we lost the Epic Yarn stage, which would have helped, but it's unfair to call Sakurai biased against Kirby for not having one stage. So again, he did the best he could with the content available to him: Magolor, the Lor Starcutter, and Galacta all made it in as trophies, MK got a Galacta-inspired alt (which wasn't great, but it was something), Kirby got his Ultra Sword Final Smash, and some Return to Dream Land and Triple Deluxe tracks were in the base-game. Bandana Waddle Dee shot up in popularity towards late 2014-early 2015, which helped in the ballot, but said ballot was never meant for Smash 4. Even then, with the DLC, we got a returning Kirby stage, with more Triple Deluxe tracks. And while BWD isn't in the base-game for Ultimate, he has perhaps the biggest chance of any Nintendo first-party right now for DLC. So it's a bit unfair to say he's been shafting the Kirby series since Brawl, since there was very little pull from in Brawl.