User blog comment:D. Cello/Assassinews 02/16 -- AC3 date released/@comment-1932060-20120222075828/@comment-1153722-20120222080954

Because they have confirmed that it will feature a new ancestor, and because if it was a Desmond-only game it would be lineair like Uncharted. Desmond doesn't have the time and security to roam around.