There are two main reasons for [using the Fabula Nova Crystallis mythos]. The first is that the world of FFXV was originally designed with these elements and links to the Fabula Nova Crystallis mythology in mind. When we came to reajust the game and change it from Versus XIII to XV, we felt it was much better to keep them in there and incorporate them in a very natural way, rather than try and remove all of them at any expense. We'd lose a lot of the world and its attraction if we did that. But why we decided to remove all the specific terminology from FFXIII was because, if you start the story and we introduce these words and ideas from the start, people will think it's still very much part of the FFXIII world. So you'd lose some of the identity as FFXV, and we really want to avoid that. Instead, we decided to not use this specific terminology and to introduce the language of the game in an easy and understandable way, that anyone can follow when they get into the game from there.
—Hajime Tabata in an interview during the 2015 Gamescom event, as recorded in English by Finaland.