Bandana Waddle Dee is a character that missed out on Smash 4. He was popular, sure, but not popular enough to even get a trophy. But his popularity has only increased, and so has his chances for this Smash. During the 8.8 Direct, we see a lot of Assist Trophies. The two that I'm going to talk about being Klaptrap and Kawasaki. Now, these both seem like they correspond with popular series that have had long-requested fighter additions. But my point being, K. Rool got in. I feel like these were almost insurance, things stopping K. Rool, and Bandana Waddle Dee, from being Assist Trophies. They aren't necessarily replacements, but almost pre-replacements. ATs that go along with the fighters.
And there's also the point of Dedede's final smash, being the Masked Dedede cage fight. Now, the only one in the audience for this fight in its souce material, Kirby Super Star Ultra, was Bandana Waddle Dee. And he's unmistakably not there in Smash. This wouldn't be too big of an issue (I get it, it's a dark stage fight, there wasn't timing for Dee to be included) BUT, This game in particular, Sakurai has been diligent about the inclusion in cameos in Final Smashes. Ness, Lucas, Mega Man, Shulk, Fox, Falco, presumedly Wolf, and even the Ice Climbers got cameos in their Final Smashes. So Dee's absense is rather glaring.