See, Disney has lots of princesses, but only a select few get to be Official Disney Princesses.
Each Official Disney Princess must meet the following requirements: A) has a primary role in a Disney/Pixar animated feature film, B) is human or mostly human-like (e.g. Ariel), and C) does not appear primarily in a sequel or commercial failure. The actual title of Princess (or equivalent) is not necessary, but certainly helps.
Although Anna, Elsa, and Moana have not been officially added to the lineup, their inclusion in the princess sequence of Wreck-It Ralph 2 all but confirms their eventual inclusion.
