Probably because of the fact that she looks extremely similar to Stella and we haven't seen or heard of Stella since the game got rebranded, unless you count that one image of all the faces from the end of the E3 2013 trailer.

Although I suppose the little girl in the beginning of that same trailer who's shown running with Noctis and the voice we hear telling him to live his own life etc. could be Stella. Or Luna. Who knows.