Why the mod can restore all the missing content but not the Aerith outfit? Is there a specific reason?
Talk:Lightning Returns: Final Fantasy XIII
Apparently, stuff like the Siegfried garb were already in the game files (and thus enabling them for use was a relatively simple matter for modders), but the Aerith garb is not, and converting things has proved troublesome. See here for more info: http://steamcommunity.com/groups/ff-modding/discussions/20/357285562479844172/
There aren't very many modders on the project and most of them are occupied with other games/projects at the moment, but the head modder says it should happen eventually.
$20 bounty to someone who can get it in the game working as intended.