I think it's less that there's not enough Modern Kirby stages to pull from, and moreso about the circumstances at the time. Brawl gave us both Dedede and Meta Knight, and at the time Kirby hadn't had any series-redefining installments that absolutely needed representing, so it was a pretty reasonable deal.
By Smash 4, we got RtDL, which defined the series for the next decade. Even so, the series was still pretty small and didn't demand a new fighter or stage immediately - Even if BWD was a popular request, he'd only been a major character for a couple games, and had only been playable once. That said, we still got Ultra Sword for Kirby's Final Smash, so it's not like Sakurai ignored it entirely. If he was really that averse to modern Kirby, he could've kept Cook, or done Mike or Crash. Sakurai's also stated that he wanted an Epic Yarn stage, but Yoshi's Wooly World was announced and that took over the yarn aesthetic, so the KEY stage was reworked into the Great cave Offensive. So, we know some of the newer games are at least on his radar.
For Ultimate, we all know new content was hella crunched, so very few series were lucky enough to have a fighter. At this point in time, we also got a few new games like Triple Deluxe, Planet Robobot, and Rainbow Curse. While decently acclaimed, they weren't megaton hits yet, so they didn't take priority for a stage. Also, Bandana Dee's only new playable appearance was in a spinoff, so he hadn't hit his current level of importance. We at least got Masked Dedede as a final smash, so that's one more piece of post-Sakurai content, even if it's just a little thing.
All things considered, it seems like Sakurai's more aware of newer Kirby games than people assume he is, and the lack of content is likely just a matter of circumstance.