She might not be a princess by birth nor by marriage, but she made an act of heroism that allows her to be in the lineup. Besides, she is largely beloved by many generations, mostly those who grew up with her. It would be unfair to remove her at this point (and tbh, it would make more sense to remove the other non-princess of the lineup, Moana, because she hasn't been there for as much time, and could possibly have a franchise of her own like Anna and Elsa).