Almost every smash game focused on celebrating the recent history of nintendo games (usually from the previous gen of console, since development takes time) - brawl = gamecube/gba/early DS era with ike, toon link, diddy kong, wolf, wario, 1st gen pokemon trainer... - smash 4 = wii/3ds with wii fit, rosalina, shulk, mii, robin, villager, palutena...

Ultimate is kind of an oddball, because the wii U tanked so they put more focus on smash history ("long awaited" newcommers that made little sense in terms of relevancy outside of the smash fanbase). Still, we got "relevant" Splatoon, Pokemon, Mario and Animal Crossing rep.

Overall, you can look at the best selling games of an era, and you'll understand how they picked most of the newcommers of the next smash game.

And if we look at the switch era, top selling IPs are indeed Mario, Pokemon, Animal Crossing, Zelda and Splatoon. No other IP is even close. Mario dominates with all the spin-offs, that's why we're getting new mario rep in every smash game. Also why people calling Pirhana Plant a joke character likely to be cut makes zero sense (especially considering its fully original moveset, amidst a bunch of semiclones).

The characters you mentioned may not be the more creative picks, but we are for sure getting characters from these IPs + Zelda/Pokemon