Likewise, this is how I feel as well.
I don't understand how Ubisoft got away with making weapons of Animal Buddies not seen in Returns, how Expresso, Candy, and Tiny can get references in Mario Odyssey, but somehow Ellie is off limits for the park.
Simple. Super Nintendo World is a generic ideal of the DK franchise, just like it is for the Mario side. Returns aesthetics are still used to represent the DK series on merch and stuff. Rambi himself is a huge part of that, being the most prominent animal buddy
SINCE he was introduced, making many cameos, being a part of Lego Mario, and being Retro's only animal buddy in the series' most recent games.
Ellie, and even something much more important and prominent than her, the Kremlings, are not a part of that generic corporate net.
YET. That's not to say they can never return, I'm sure they will, at least outside the parks, but look at it this way. Sonic, as a series has many characters and locations that people adore and want to see return. A lot of them even get referenced in Sonic Frontiers, despite not actually showing up. Quite a few of them have ended up showing up in merch or mobile games, but a lot of Sonic merch, toys, shows, the movies, etc, start with the basics. Sonic, Tails, Knuckles, Amy and Eggman in Green Hill Zone and they basically build from there. Until DK's in a more stable state, it'll basically just be the starting cast.
The parks here basically have to be as timeless as possible, and even though going with Tiki Tong
IS strange for that reason, it is a part of the most recent and iconic depiction of standard DKC. The Tiki Tak Tribe is in Nintendo's second best selling game of all time. The Kremlings, sadly, are not.