Exactly. People have wanted Dixie Kong and Bandana Dee for years at this point and Sakurai still refuses to add them.
To be fair, Dixie was not added in favor of King K Rool, fans made a big noise for that character. Though, it's less a matter of who and more a matter of how much content from those series in general (not just characters), and the balance in representation. Kirby has got quite some stuff with the Marx boss and some items and stages, but all of them come from the usual 4 games, Kirby's Dream Land, Kirby's Adventure, Kirby Superstar and Kirby Air Ride (that, coincidence or not, are the only games that Sakurai directed himself); other games rarely get referenced... it took 2 Smash games of people asking to make Dedede's final smash Masked Dedede.
Adding one character is not enough, they added K Rool and DK is still missing Dixie, if they added Dixie people would have complained of no K Rool.
They should add enough characters to each series to make them feel complete... and don't let me start on the Bandana Dee debate.
There are many Kirby characters who would be very fun to play as, and people are still obsessed with the one who weilds a traditional weapon. The Dark Matter trilogy is still not represented aside of a few spirits and a couple of songs (2 low effort ports, one is a remix but it was made more than a decade ago, the other is an N64 midi port). Modern Kirby has much more to offer than just Bandana Waddle Dee, too.
In my opinion Kirby would be perfect with 6 characters, 5 originals and 1 echo (obviously Galacta Knight); for the 5 originals, I would personally add Magolor and Adeleine, one to represent modern kirby (and final bosses), and one to represent the Dark Matter trilogy (and several classic enemies including Kracko). People want Bandana Dee, so Bandana Dee and Adeleine would be a nice compromise to make everyone happy, but I really think that Magolor would be a much better modern rep than Bandana Dee (I think Dee would work better as one of those advanced assist trophies than as an actual character, and Rick, Kine and Coo have been important enough in the history of the franchise to at least deserve an assist trophy too, they even got a spinoff series in Kirby Star Stacker).
Technically, the Kirby series is one of those where you have lots of options, heck even freaking Chef Kawasaki would be an interesting fighter if tweaked in the right way; too bad it only has 3 characters.
For DK, it's a lot easier. Just add Dixie, Cranky (yes) and echo Funky, and that's all the series needs characters-wise. 6 (5+1) characters one, 6 (5+1) characters the other... it will probably never happen because it's more likely that a reboot comes first and we'll get less characters instead of more. (I would also take a reboot as long as it actually improves the game and brings innovation; I'm all for quality over quantity).