Yeah... I could think of a number of different reasons but I will only talk about a few but I don't want to be here all day so here it goes, it's just not many people are going to like them.
Why did they not show Ridley during the direct during the boss discussion? First of all, it could be very possible that Sakurai and his team wanted to show off the boss concept but for whatever reason decided not that they didn't want to actually show the bosses entirely so they went with Ridley which would be the easiest to recognize without showing him directly. Second it's possible it was mere dramatics and nothing else, the scene is an obvious play on the horrible monsters in the shadow display by tailing the other character, I admit the execution isn't particular great do to the upbeat nature of the direct but there you go. Another possibility is technical reasons and not for the 'isn't finished' reason you were talking about before. It could be very possible there were other kinds of issues with the Pyrosphere stage or even Ridley himself as it is something that happens because weird things do happen in programming a lot of the time, sometimes things that can happen last minute in a new implementation or alternative build. That doesn't make the team incompetent, it's just one of the more frustrating aspects of program development of any kind.
As far as since then, since Sakurai has mentioned almost nothing about the bosses outside of the Yellow Devil and the Emperor, Sakurai is simply not talking about them for whatever reason, prehaps he simply doesn't want to reveal them before the game is released. Same thing happened in Brawl if I recall right, Ridley's bosses appearance weren't known till after Brawls release. There is one other possibility I can think of but it's the possibility that no one here is going to like. It may just simply be that Sakurai and his team simply don't consider Ridley important enough to spent a time and detail to talk about in general, that they may simply not care enough to bother. For all of our luster and devotion for the character may mean jack squat beyond the role they chose for him to those actually making the game.
I don't want you to misunderstand, I do want Ridley in the game, but it's very hard for me to accept this notion that based on speculation that Sakurai was either lying or trolling without any actual evidence that he was and until something comes from the development team or Nintendo to suggest otherwise, I can't accept the speculation as some kind of fact or believe he's playable strictly from those notions..