Because from the things that Nomura mentioned from the latest interview about adding more FF characters, more Org. XIII members (human), or (could possibly) more worlds, they didn't made it to the final cut.
And since there wouldn't be anymore game that takes place in the past & have that kind of volume (except for BbS), it would be extremely necessary to have a BbS Final Mix (eventhough all of you guys says only the Main Games could have the Final Mix treatment).
So therefore, by adding the FF characters (Cloud & Leon's backstory), adding the remaining members (Demyx, Luxord, Marluxia & Larxene in human) and worlds (like what people were hoping for BbS is Atlantica), the mysteries of BbS will be solved 100% full of complete info of the past.
Then also, if they could try to add a "fourth scenario," we could explore King Mickey's time before, during and after the story of BbS (setting the stage for KH1).
And finally, add more add-ons to the gameplay (such as D-Links, Styles, Decks, Boards, Enemies, etc.)
EDIT: The final mix should be similar to Dissidia Final Fantasy: Universal Tuning, a re-release of Dissidia to the same handheld, PSP.