Tiana's quest was too adult
In Disney Princess films, the princess is on a quest to save family (Mulan and Belle) or make things better (Pocahontas) or trying to find family (Tangled) and in the process finds herself and true love. Tiana found herself in search of the money to open up her own business (save her father's dream). Although, it had an emotional element to the quest (save her father's dream), the pursuit owning one's own business is a little too sophisticated for the little girls. Now had she been on quest to save her people, or rescue her family or father, then that's something the little one's can understand.
There is new movie called Belle (about a black girl being raised by her British relatives (her white father's family) after her black slave mother dies. In the movie Belle under goes a lot of difficulties due to the fact that she is black living in England during the time of slavery. Belle uses her influence with her uncle to eventually cause him to release some slaves. I have not seen the movie, however, her uncles actions eventually lead England to outlawing slavery. During her journey she finds true love and learns a lot about herself. Now that would have been a great Disney movie.