I really don't think there's any explanation necessary anymore about FFXIII's failure when the team itself was fully aware and admitted to the troubled development in the post-mortem.
Lack Of A Shared Vision
Although the game was announced with an elaborate trailer, the team had little direction beyond this proof of concept, which led to problems as development ramped up.
"Final Fantasy XIII was first introduced through a concept trailer shown alongside the announcement of the Fabula Nova Crystallis project at E3 2006 (Fabula Nova Crystallis represents a suite of games and other entertainment media related to Final Fantasy XIII). The trailer was merely a visual concept, and we had not yet created anything playable at that point.
I felt that this trailer set the bar for the quality we were aiming to achieve, in terms of battle speed and cutscene imagery, and believed that this sentiment was shared by the rest of the team.
However, it became clear that, at the time, there were actually very few members who saw the trailer as a representation of what we wanted to achieve with Final Fantasy XIII. This lack of a shared vision became the root of many conflicts that arose later in development."
[...]
"Even at a late stage of development, we did not agree on key elements of the game, which stemmed from the lack of a cohesive vision, the lack of finalized specs, and the remaining problems with communication between departments.
What enabled us to conquer this line of seemingly endless conflicts was the development process for the Final Fantasy XIII demo, which was included in the Japan-only Blu-ray version of the animated film Final Fantasy VII: Advent Children Complete. The demo was not in our original plan, so we had to make adjustments to the overall schedule to accommodate it. Whatever effects creating the demo had on the schedule, once it was complete we realized it was just the panacea we needed.
[...]
More at the link:
http://www.gamasutra.com/view/news/...Scenes_Of_Square_Enixs_Final_Fantasy_XIII.php