Bad luck has unfortunately been a part of Dixie Kong's identity going back to 1997 (where she was not a part of Diddy Kong Racing's roster).
Every single thing you could think of that would be a point in her favor to be added to the roster, turned out to be a mirage. Heck, both DKC2 and DCK3 were among the Top 10 best selling games for the entire SNES/Super Famicom. Heck, even DKC3 sold about two million copies in Japan alone.
Everything that could lead you to think there was hope for her ended up being a mirage:
- planned for Brawl
- unveiled to have a major playable role in Tropical Freeze at around the same time as Smash 4 was unveiled.
- Diddy Kong lost his tail attack
- No new Tropical Freeze music in Ultimate as well as the weird inclusion of Ice Cave Chant
She is a fantastic gaming character, and among the few heroines that has a "Mario appeal" (meaning a character that is appealing and endearing across demographics despite having an unorthodox mass appeal design). Here is a post I wrote recently explaining this:
During the 1990's, there were very few female gaming protagonists, especially outside of RPGs. For Nintendo, the two big ones were Samus (who many did not even know was a woman) and of course Dixie Kong.
Dixie Kong was a very interesting case. For many female characters, they are usually loved because of "sex appeal" or "waifu bait". However, Dixie Kong was a very "girly girl" kind of character (she would even cry if she got hit), yet she was very popular among boys during the 1990's. Why was this? Well, even though she was still very feminine and girly, she was at the same time very cool. She could fly through stages that were very challenging for our much younger selves, and flying is very cool. Her animations did much to endear people to her too, whether it was rocking out on a guitar, chewing gum, having a drink through a straw, or licking honey while climbing a wall. She had spunk and personality. DKC3 might very well been the first Nintendo game I remember ever being hyped for solely due to Dixie Kong headlining it (it was either that or Yoshi's Island).
Dixie Kong in a lot of ways is a female Mario. You would not think a girly monkey with an oversized ponytail would be very endearing (much the same way you would not think a mustachioed middle aged man wearing a red cap and shirt and blue overalls would be appealing), yet she is appealing and endearing to all kinds of demographics. She in many ways is the ideal heroine. Many heroines are made and written to be men in a female body, but Dixie Kong is still very feminine and a "girly girl", yet at the same time very much a heroine. For a series that regularly features kidnapping as a plotpoint, she still remains the only major playable Kong to have never been kidnapped by K. Rool (DK and Diddy have both been kidnapped twice). She has fight in her too (remember how she sassed K. Rool in DKC3).
Young boys would love and appreciate Dixie Kong just as much as young girls would, and that speaks a lot to her design and appeal as a character.