Because Dissidia 012 shows the battle before Dissidia. And also, if you recall, the original Dissidia was the end of the battle. it would be 011 because it would be the eleventh cycle. There wouldn't be an 014, because the battle would be over, so there would be no storyline, unless you follow the characters through their stories. that would take too much programming.
It would be easier to add in a handful of new characters and add a few new abilities/attacks and stages.
Yes I know I am a few years late, but I just happened across it now.