The princesses are definitely a big part of Disney - but how would they really fit into this game? I could see them trying to fit the characters in somehow, but not as playables. Even in Kingdom Hearts, where they changed up some Disney characters and gave them fighting skills, the princesses did not (except Ariel, who'd have a bit of trouble walking around unless they used her human form). Not only are the characters simply not fighters, it would be against their characters I think. And with no combat, how would the characters operate? That's a large portion of the game. Even Monsters U has the characters throwing things at each other, body slamming, and the paintball battles. And even if the princesses got their own playset with no combat, remember that any player character can use the tools & packs in the toy box.
Picture them releasing a trailer for playable princesses, and starting it with Cinderella pulling up in the Incredicar, then jumping out with a scimitar in one hand and a laser blaster in the other. Yeah, there'd be a certain awesomeness to it, but don't you think it would be weird?