They didn't go back and get Rosalina & Luma though. They were added because they were the relevant characters at the time. Same thing with Sheik and Zero Suit Samus.
Galaxy came out about 7 years prior to Smash 4, her latest appearance being a secret unlockable character at the end of 3D World. Before then, just the 120 stars ending of Galaxy 2 and the end of its true final level (joining Starship Mario only after the ending or something; though she also sent letters to the baby Luma without revealing it’s her I guess, and there’s the Cosmic Spirit but it only appears if you die too much), and playable in spinoffs. If that’s enough for relevancy at the time, Hyrule Warriors should count for Ultimate, or at least a BotW character besides the latest incarnation of Link could’ve been added.
There’s also the 3DS and Wii U remakes/HD ports bringing some characters back in the spotlight for more people to see, but admittedly those may not count for whatever weird reason.
While the main Zelda cast is pretty much just the Triforce-wielders, a lot of times Impa, and sometimes Tingle, they at least also seem to see importance and popularity in other characters of the series, like Skull Kid and Midna, so...and of course, there’s still Impa and Tingle themselves :/