For being on the backburner for so long, I don't think anyone really knows. I know that the First Production Team of SE is in charge of Dissidia, Kingdom Hearts, Fabula Nova Crystallis, and Compilation of FFVII. All of which have seen quite a few releases in those 6 years.
Could they all of been in development before Versus? I doubt it, but I wont rule it out.
Sony wanted other things pushed first? Possibly
Maybe they were going to have a tie in with FFXIII - 2 with all the paradoxes, but scrapped it.
Also the luminous engine is confirmed to be the lighting engine for the game, while gameplay will be a completely different engine. Perhaps they were just running with a beta or alpha version of the engine all this time. Since its just been recently announced (i think also in sept 2011) and we just barley got a tech demo for it at E3.
Lastly, since they are owned by Sony and probably the most important developer to Sony, they would probably have first knowledge of a new system.
I could really see this being the reason. As the PS4 isn't announced and probably wont be for some time. They can't reveal anything new about the game because.... well its going to release with the PS4. It has the engine, story, whats left? A release date.
Reasons why it may be retitled to XV.
It literally has nothing to do with FFXIII / - 2 or Typo 0 other than its set in the same universe. Which could literally mean nothing. It could be a completely independent planet set in the same universe. Wikipedia does mention that it has the same mythology though.
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