So, here are the reasons:
She has less with the story than characters like Snow and Sahz, their interaction with the party members (Hope, Vanille) is much more important then Lightning's, she also has less to do with the villians (Rosch - Snow and Big Boob hot lady - Sahz), also, when you check story's likr FF7, or 10, you see that the main character is someone that the story revolves around, someone who's in the center of the plot, Lightning isn't, she could easily be replaced iwth someone else, without the same backstory, and FF13 will be almost the same.
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