Because the Templar conspiracy wasn't around until Altair's time during the first game.Banok said:why does the series have to go forward in time each game? that would be lame.
The 3rd game could be set before the 1st, much more interesting time periods IMO.
one of the things that originally pulled me to AC1 is because of the medievil setting. hell set it in ancient rome or greece. WW2 FTL.