I just wanted to point out the possibilities that Sakurai chooses when deciding newcomers, maybe these reasons will determine something!
1.
Promotion! Promotion is one of the biggest reasons. Sakurai most likely wants characters that have a high and dedicated following, as well as make the character appeal to almost everyone as possible.
(Support - Promotion & veteran returns: Rosalina, Greninja, Mewtwo, Lucina, Robin, Palutena, Toon Link, & Lucario)
2.
Uniqueness! Sakurai wants fighters that are unique. Bandana Dee has an advantage here, he's not a boxer/wrestler and yes he may have a spear somewhat similar to a sword... But Kirby games (ex. Triple Deluxe & Kirby Fighters + DLC) seem to take some inspiration from Smash and make copy abilities have a good variety of moves.
And not just unique move-sets for abilities, but characters in the Kirby series as well! Meta Knight's unique ability is to glide & Galaxia, King DeDeDe has an unique gigantic hammer, Kirby is Kirby, and Bandana Dee has an awesome bandana, as well as a agile spear (That can also hover)!
All in all, spears are not highly as used as swords in the Smash series, or in general. So Dee's unique, and short yet agile spear brings many possibilities, tricks, combinations, and something new to the table.
3.
Protagonists! Protagonists seem to matter a lot more to Sakurai than villains, sadly. But, considering that King DeDeDe in some cases is not really considered a villain any more, Meta Knight is strongly towards the good side, Mewtwo is a 50/50, and how Wario is pretty much an anti-hero, we can conclude that only "true" villains are Bowser, Bowser Jr. (& The Koopalings), and Ganondorf. There is also arguably Dark Pit and a spoiler from a series revolving around many sword fighters.
Anyways, Bandana Dee has become an outcast villain to an ally, and friend to Kirby. How can anyone argue
not include Kirby's friend?
4.
Anniversaries! Anniversaries do play a factor. While this may not be as big as a point, some veterans, such as Toon Link, was not only announced, but also promoted from The Wind Waker HD. I do believe some stages, assist trophies, and characters in general were announced on yearly anniversaries. Why do stages & assist trophies matter? Anything helps in this case! From veterans being announced on special days to content being announced around certain times as well, anniversaries do play somewhat of a factor to Sakurai.
5.
Sakurai. It's Sakurai. You cannot predict this man. Yes, you can make assumptions and your hardest predictions, but in the end we get Duck Hunt, Mr. Game & Watch, R.O.B., Wii Fit Trainer, and Dark Pit.
On the positive side, any character has a chance. Big, small, popular, non-popular, villain, hero, main-character, side-character, strong following, no following, the list goes on.
What I'm trying to say is, is that any character has a chance to become a fighter. King K. Rool realistically has as much of a chance as say Sakurai's cat. Ok, maybe I'm going to far, but you get the point.
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In conclusion, yes, you may have heard some to all these reasons before, but I think everyone needs a reminder that anyone can become an awesome, beloved fighter across all fan-bases and the sometimes questionable decisions Sakurai makes. Overall, Bandana Dee has advantages of being a very likable protagonist, he's recognizable, unique, and the Kirby series was once Sakurai's baby. And deep down he knows it still is.