Honestly a big reason Dixie isn't in yet is likely the lack of a DK newcomer in Melee, presumably due to the Rare debacle.
I actually think even if Diddy Kong was added in Melee, there is a good chance that Brawl would have had no DK newcomers (and neither would have Smash 4). Keep in mind, Dixie Kong did not start to make appearances in the DK series again until Donkey Konga 2's release in 2004, and K. Rool did not start having antagonist roles in titles again until 2005's DK: King of Swing. Keeping in mind that most of Brawl's roster was decided in mid 2005, I think there is a very good chance that neither Dixie Kong or K. Rool would have been a part of it. Dixie Kong and K. Rool having no new appearances after 2008's Mario Super Sluggers was probably the main factor that kept them off Smash 4's newcomer list (all of Smash 4's newcomers had a prominent software release after Brawl with the exception of Duck Hunt).
Dixie Kong is a character that historically has had bad timing and bad circumstances in regards to Smash Bros. She could have been retooled as a separate semi-clone of Diddy Kong back during Brawl, but was not. She probably would have been part of Smash 4's roster had it started development a year later (where Sakurai would then had known about her upcoming role in Tropical Freeze). K. Rool dominating the Smash ballot and there being only a small list of newcomer for Ultimate (of which only the most elite of newcomer choices, with the exception of Incineroar, Piranha Plant, and the echoes were able to make the cut) meant she simply could not make the cut. And now Ultimate going with very likely all third-party characters for its DLC.
It is pretty ironic how the three characters with the biggest claims to being Nintendo All-Stars left (Toad, Dixie Kong, Bandana Dee) all have unfortunate circumstances surrounding them that normally should not have kept them out.
Ridley was most likely the "everything is possible" newcomer since he had the most going against his inclusion in Smash.
I actually agree with this (partially). As a very long-time Ridley fan and supporter, I honestly thought he was most likely never going to be playable in an official Smash Bros. title after Sakurai came out and said he thought Ridley was "too big". It was simply a miracle that Sakurai re-considered his opinion on Ridley so soon after he said those thoughts on him (only a year later). Sakurai has gone on the record to say Ridley was among the most challenging characters he ever had to make, so it was definitely a miracle even through a design stand point.
The other miracle associated with Ultimate was simply bringing every single veteran in the history of the series back. I never thought this would happen (especially considering how some veterans such as Pichu, Young Link, Squirtle, and Ivysaur seemed to have relatively few championing their return). Not to mention all the arm wrestling it probably took to get the likes of Snake and Cloud on the roster again.
There are also other newcomer additions to Ultimate that were miraculous. K. Rool's inclusion was likely due to his ballot performance and probably would never had made it in had there not been one. Both Dark Samus and Chrom also were miraculous inclusions. Dark Samus's last game appearance was all the way back in 2007 and she did not seem to have many requests for her (she was also the only Assist Trophy to be promoted aside from Little Mac and Isabelle, both of whom are Nintendo All-Stars), and Chrom hails from a series where it typically is the freshest face that is chosen to be the newcomer for Smash Bros. and the fact that his game of origin already had two playable characters already).
All in all, Ultimate granted a lot of miracles in regards to character selection. I said before that Ridley, K. Rool, Simon, Inkling, and Isabelle were the best of the best as far as newcomers that they could have chosen. The echo selections were also fantastic. Even Incineroar and Piranha Plant had strong points in regards to their inclusion with Incineroar finally bringing the pro-wrestler archetype to Smash Bros., and Piranha Plant upending expectations for what and who a fighter can be.
It is simply incredible.
I personally would have really liked to have seen Bandana Dee and Dixie Kong on the roster (they are the only two Nintendo All-Star characters left that do not have some sort of major role in Ultimate and their inclusion would probably make at least the Nintendo side of things seem "complete"), but on the whole we got a really fantastic roster that a year ago I would have deemed extremely unlikely to ever happen.
Mogisthelioma
said. For DK, what they said plus the fact that Dixie was supposed to be in Brawl and if we had K. Rool in Smash 4, we very well could have gotten Dixie, probably as an echo/semi-clone. As for Zelda, most of the characters from that series are one-offs with none of them being that important to the series, besides Impa who could be in, but Sakurai might know her for her appearances that don't resemble a Ninja or just can't envision her fighting like other characters, since only HW Impa actually fights, who Nintendo's not able to get due to copyright. As for Modern Kirby, there's no reason. Why pick Marx for a boss when you could have just picked Magolor or Galacta Knight? Why make what was supposed to be Epic Yarn one of your games and not a game w/o representation like RtDL, 64 or Squeak Squad? Why make absolutely ZERO modern Kirby remixes while 2 of Sakurai's games get one?
, I'm proud of my boi.