Sakurai said during Brawl's development about Meta knight inclusion: "I knew the character (Meta knight) was popular in Japan, but last time around I held off and didn't include him in the game. I wanted to put other people's titles ahead of my own". (Source:
https://www.gamezone.com/originals/sakurai-responds-to-ssb-brawl-poll/ )
We also know that King dedede was cut during Smash 64 development. Maybe it was only because of time restriction or system limitations, but I feel like King dedede would have been, at least, considered in Melee after that and just like Meta knight, held off for the next game.
Also you could note that a lot of criticism went to Sakurai after Smash 4 because of the "over representation" of Kid icarus. If I had to guess, I think this is why we don't have Bandana waddle dee playable; Sakurai was afraid back in Melee to add his own characters, in Brawl and Smash 4 he changed his mind, but when he add Dark pit people criticize it, so now he don't want to take the risk of adding other of his character and getting criticized again. That's just a big theory, maybe there's totally other reason about why Dee isn't playable.