This is a question I see asked occasionally. While I don’t have the “official” answer, here is my theory:
Keep in mind that Disney is about bringing magic to its guests. This of course includes the “Magic Kingdom” where you can relive their amazing stories in an interactive environment.
Both Cinderella’s Castle and the Beast’s Castle are keeping within a theme. While Cinderella’s castle is done post-transformation when Cinderella turns into a princess, the Beast’s castle is pre-transformation from before he was with Belle or he wouldn’t still be the Beast and the castle would look completely different.
The Beast lived a secluded life before Belle. Remember, the people in town had been put under a spell to forget he was there (which we are given details about in the live-action film). That means the only characters that would logically be a part of that scenario would be those of the enchanted objects, such as Mrs. Potts, Lumiere, Cogsworth, Chip, etc. In keeping with the theme, these would have to be puppet sized as they are in Enchanted Tales with Belle. Adding any other characters outside of that would taint the experience.
The Beast only comes out at night because, again, that’s in keeping with his character at the time he was still under the spell. He’s supposed to be secretive, dark, mysterious, and very self-conscious of his appearance.
Why not at least Belle? Because then we are interrupting the storyline of the time they fell in love. This is a precious and private moment in the story which is to be experienced at Enchanted Tales with Belle in a more cozy setting. This is also, incidentally, where the enchanted objects are.
The whole experience is designed to be as close to meeting the characters in a natural, logical setting as possible, which is also why you’ll only find Gaston in Storybook village. The whole package is designed to work in harmony as individual pieces of the story. I think they’ve done a lovely and perfect job of it. I, for one, wouldn’t have it any other way.
Again, I do not represent Disney in any way. I’m merely a lifetime fan. But, it seems a pretty straightforward and logical reasoning. Please share if you have any thoughts!