Dixie Kong, to me, is a very unimportant character. She appeared in Donkey Kong Country 2 (later than Cranky, Rambi, Squawks, Enguarde, Winky, Expresso) as the second playable character, then in DKC3 as the protagonist. The only characters that were playable (in a sense) in all three countries were Rambi and Enguarde. Characters that weren't playable but appeared in all three games include Donkey Kong, Diddy Kong, Cranky Kong, Funky Kong, and Squawks.
After the Donkey Kong Country series, Dixie was replaced for Tiny Kong, who is now known as her younger sister. For a while, Dixie was off the radar and generally replaced, appearing in cameos and some Mario Sports games. Other than the two games she mattered in, Dixie Kong really hasn't been as prominent as the Animal Buddies.
Animal Buddies, in some combination, have always been a part of the Donkey Kong Contry series. They have been a bigger part of the series as a whole than Donkey Kong himself! Rambi specifically hasn't missed a game, and that includes DKCR! He was also in DK64 and others! Personally, I see Rambi alone as a more important aspect to the Donkey Kong franchise than K. Rool and the kremlings! (And I'm a huge fan of the kremling krew to begin with).
The other Animal Buddy that has appeared in all of the country games plus DK64 is Squawks. Squawks is certainly the next most important Animal Buddy after Rambi, no contest. Enguarde comes a close third but, unfortunately, he is a water dweller and could not fight on the land stages as the swordfish he is.
Next I began to consider the correlation to the smash series, and integrating these obviously important animal buddies as a character. I pretty easily saw a Pokémon Trainer-esque character, consisting of a lightweight (Squawks), and a heavyweight (Rambi). Next I needed a middleweight. I chose Squitter the spider.
Squitter is obviously less well-known, less seen, and less important than the previous two choices. However, I chose him not from personal bias, but for the unique and interesting playstyle he could bring to smash. He was also one of the only obvious middlewieght animal buddies that could function well as a fighter. Using his web platforms to literally make new sections of land to stand on (and maybe even tether recover onto) and shooting web balls out at opponents gifts Squitter with something cool and interesting that no other fighter can do anything close to. He was the perfect contender for a slot as I saw it.
Cranky Kong has probably appeared in more Donkey Kong games in general than any other Donkey Kong character.
From the original Donkey Kong game (where he was actually the first DK who kidnapped Pauline and threw the barrels) to all the countries, to 64, to the cartoon, to any Donkey Kong game that exists, you could be sure Cranky was in there somewhere. While he is rarely playable, his job in smash would be nothing strenuous. Basically, Cranky Kong is the trainer and the animal buddies are his pokémon. From the safety of the back platform Cranky Kong would sit in his rocking chair or stand with his cane and tell his animal buddies what to do. It stands to reason that if he was the original Donkey Kong, he himself forged the bond with the other animals of the jungle. Squawks is even helping him work in his shop in DKCR!
Alright, I lost track of where I was going with this and I think it turned more into how the character would work than why it should happen. Sorry for the wall of text. I seriously believe Cranky and the Animal Buddies are just as important or more important than K. Rool and Dixie Kong. Of the three characters, which were represented in the more recent Donkey Kong game? That's right, Cranky and his animal buddies
TL;DR - Cranky Kong commanding the animal buddies as a character is more recent, more unique with a better moveset potential, and more "important" to the series than Dixie Kong, and maybe even K. Rool (although he WAS the main antagonist for a time).
