I said this in another comment the other day, I think they want or are maybe obliged to keep some of the mystique around the isu.

If they gave us the Isu apocalypse storyline exactly as it happened with no filter, they've basically given closure on that storyline and there's nowhere for them to go after that, other than maybe back to the assassin/templar struggle which can only remain interesting for so long. So basically it would end the series and that means no more yearly injection of cash for the publishers.

I also think ubisoft are clearly bored of these games and want to do something else, but again the publishers and how lucrative the series is has then locked into making more of them. So they're pushing the series as much as they can in the direction they want to go while still slapping the AC title on it.