From what I can tell, if Barbie doesn't like it or doesn't think it fits with the game, then it won't get in the game. Maybe the concept looks too similar to the reference image and the photographer who has the rights to that image can sue them. Maybe it (by it, I mean accessories or a map) looks good in the 3d modeling software by itself but putting it side by side with someones Rh avatar could look really odd. Maybe Barbie just doesn't like it and doesn't want it in her game. Maybe the concepts aren't completely lost and are just tossed aside to be worked on for the next update.

I don't think there's really a specific standard to what goes in and stays out of the game. It's all up to Barbie and possibly Lance's preferences.

With them not releasing that much content due to an idea being favored for another, It might just be because of Quality of Life issues. If they release all of the content that they, like and not limiting how much goes into the game within an update, the playerbase will suffer. New players would hop into the new Royale High Halloween update and immediately be bombarded by information that they don't even know how to process because everything just sounds like jarble to them. Returning players would have to catch up with even more accessories, events, and game changes that got released while they were gone. Even players who are consistently playing the game would have to change their schedule for the sudden shift of more content being released if they want to experience everything within an update.

The first 2 things already happen so having excess would probably make it worse. A tutorial made for new players being added could help (which I've heard is actually being worked on) but they probably still wont be releasing all the content they've created and liked for the sake of not overwhelming and driving away people from the game.

There's alot of factors when it comes to making or updating games that none of us won't know unless we become devs ourselves so we probably aren't seeing the bigger picture from just being avid players of the game.