Actually, barring the likes of Pit and R.O.B., Brawl did the same exact thing.
It's just less obvious because a lot of the more relevant stuff happened to be characters that have existed for longer.
More times than not, rosters are majority contemporary.
Disclaimer and sorry in advance but I ended up typing out more than I expected here, kind of just an assessment of why people see it this way.
I think Brawl landed at a sweet spot where many of the most glaring omissions were still on the table and those series were actively getting games. When we look back at that roster we think of Wario, Diddy Kong, King Dedede, Meta Knight, Pokemon Trainer... all pretty evergreen picks. A lot of the characters who fill that role in their series have had plenty of opportunity to be added by now, outside of some series that were added to Smash a bit later and are either still in their relative infancy or playing catch-up. Now we've got Ridley and Isabelle who are right on par with these other characters. But the bottom line is nobody would ever deny the continued significance of most of these Brawl adds.
But then we had Ike, Lucario, Olimar, Lucas... three out of four of these have aged tremendously well, but were brand spanking new at the time. Ike and Lucario are some of the most popular characters in their respective series to this day and Pikmin has established itself much more confidently. Lucas is beloved but well... I guess that's one instance of a character who really would have had no shot in any other game. One of the only examples of that in Brawl. I harped on this the other day so I won't get too deep into it but I suppose Zero Suit Samus has aged kinda poorly relatively speaking.
Nowadays it's pretty controversial to suggest that even major series like Animal Crossing or Splatoon deserve more than the bare minimum. I don't think some of these series will ever shake the "new Nintendo" stigma when the fans are getting older and still holding onto picks from a handful of longstanding series instead, or older unrepresented IPs. So later rosters are gonna feel heavy on the new stuff, something Smash 4 is often criticized for, but it's really just a combination of the earlier games laying the necessary framework already and many veteran community members having trouble keeping up with or accepting where Nintendo is today. Surely the likes of Tom Nook or Squid Sisters fare about as well as many of the persistently relevant characters from Brawl.
In the grand scheme of things, I think the community has become unnecessarily jaded or their expectations warped to believe a roster is just going to pile on a bunch of characters who debuted in a few Switch games and have like one appearance to their name. The ""Fire Emblem Situation"" has certainly contributed to this as well as the pattern of Pokemon and this growing boogeyman / agenda of the same thing happening with Xenoblade. I think we need to take a step back and recognize that Smash has always juggled the contemporary with the evergreen. Villager, Shulk, Miis, Bowser Jr and Greninja have all aged pretty gracefully too. And no reason to even touch on Ultimate's roster really, it was hardly a problem there and I hesitate to factor DLC in too heavily.