How so?
Well, Renmiri makes a good point.
There was a scene right as the party is leaving (SPOILER)Jahara where Vaan and Ashe have a really cool moment where Vaan states that he 'is ready to find some answers. He then spends the rest of the game not doing that, even though you would think he would, you know, at least ask Basch about some stuff or something, since Basch was, you know, there and everything. So, this subplot gets dumped, but we go right into a little of Fran's story, which was interesting and all, but then it gets dropped right when you leave the area and never gets picked up again. (SPOILER)Why exactly did Fran leave Eryut Villiage? I am not talking about some generic 'I wanted to explore the world,' I want to know (SPOILER)why she decided to leave her family and everyone she loved, including the Wood, to partner with a Hume, when, by all accounts, Humes are distrusted by the Viera.
There are a lot of things I would love to know about the characters and Ivalice, it makes it all the more frustrating the game didn't explore these things more.
So that is why I think the characters are underdeveloped.