In order:
Pokemon has yet to skip a game without a new rep. And I don't think it's ever gonna start doing that.
Isabelle/Tom Nook are some of the most iconic and successful remaining All Stars and bot have facets Villager can't do. Isabelle in particular is the face/mascot of the franchise since her inception. You guys are just mean.
Mario is the mascot of Nintendo and has always has someone that somebody out there is gonna recognize. Everyone clamored for has some merits and probably placed well in the ballot because of the sheer size of this franchise worldwide.
K. Rool probably did fairly well in the ballot. Dixie was considered at one point and cut for some reason, so I assume Sakurai has a moveset in mind with her and it most likely wasn't similar to Diddy at all seeing as she was his tag team partner to represent the old DK swap system.
Fire Emblem is still obnoxiously popular despite being having way too many reps as it is right now. It might see a scale back on reps and get someone new added in.
Ridley is a true dark horse, but one of the most asked for one people in all of Smash history. Sylux is an even a possibility if he truly does have a big role in Prime 4 so it might be advanced promotion to put him in. So maybe here.
Kirby's three most iconic individuals are all already in the game. Sakurai seems to prefer his era on the games and be a bit like Miyamoto when it comes to any games after his vacating the series. Thus I don't see Kirby getting anyone but if it does it'll piss off everyone who likes a certain spear wielder.
Xenoblade is just....well, it's Xenoblade. I think everyone on the internet is overselling how much it means to Nintendo now. It's still very niche, but growing. It's only saving grace is that it remains one of the few Nintendo IPs to be doing so right now. Yet when it comes to people to add? Any of it's possibilities just don't seem as if they'd be very high priority for Sakurai with all the potential he has elsewhere.
Zelda is stuck in it's rut, because either you have Impa who can sit out games or a polarizing figure like Tingle to add. Everyone else is just a really really cool guest person who showed up once or twice and I honestly think Sakurai just isn't in favor of this franchise since he's taken to Ganondorf being a punchy and kicky dude despite being a sorcerer in his games.