Not only is Bandana Waddle Dee literally a Waddle Dee wearing a bandana which is a Goombario tier design and totally unsuited to being a serious character for that alone, but he sure wasn't in any of the Kirby games I played which is I think all of them up through Kirby 64. I know there are a ton of newer Kirby games and maybe he's important in some of them, but the ones of them I've seen substantial gameplay of didn't really feature him either though I can't promise he didn't make a cameo somewhere (Super Star Ultra, Epic Yarn). If you are a major Kirby fan and play every last game in the series and are super focused on specific newer titles maybe he seems really important, but I think you'd discover that to a lot of people who have played plenty of Kirby games he's still a nobody. Comparing to Luigi is silly. Comparing to even Toad is a serious downplay of Toad.
All of that aside though, when it comes to characters who are not household names, you have to remember that a lot of it comes down to selling the character to people who don't know who the character is which requires an interesting design. K.Rool is very obscure by Smash character standards, but his design is complex and extremely expressive which made it pretty easy to make him look exciting in a trailer (and I'd point out the hero vs villain montage was probably mostly just to inform a lot of younger players who he even is; they wouldn't have instantly guessed he was a longtime DK enemy). K.Rool's history also involves him doing a ton of really interesting things which made giving him a diverse moveset with tons of really unique moves really natural. Imagine trying to sell a design that is a Waddle Dee wearing a bandana carrying a toothpick that is supposed to pass for a spear. It would really be like trying to make a hype Goombario trailer. Now here we have... a Goomba! Remember those Mario players? This one is special! He wears a hat! Look at his thrilling moveset of walking around using his one and only means of attack in 15-20 different ways to constitute a full Smash moveset! Isn't that almost but not quite as full of flavor and diversity as what Jigglypuff, the current least complex Smash design, can do? Buy today!
Yeah, that's appealing to no one. The only way he gets in is if his basic design is so re-imagined that he's pretty much not the same character anymore. Remember we've already had Waddle Dees in Smash. They were a King Dedede accessory just like Toad is a Peach accessory. Unlike Toad, who in the newest game is a more prominent Toad accessory, Waddle Dee was unimportant enough to disappear from King Dedede's moveset. I can't promise for sure anyone is or is not in so maybe serious Kirby favoritism will win the day for him, but of all common suggestions, Bandana Waddle Dee strikes me by far as the least likely and frankly the worst decision they could make. He's impossible to sell to people who were not already Kirby fanatics; that's instantly disqualifying or at least sure should be.