Rome is built on history. Since Assassin's Creed is all about the history of the Templar's and Assassin's, placing the game within the Roman Empire is perfect. Julius Caesar, Brutus, Cleopatra, Marc Antony; there are plenty of well known and powerful faces that our Assassin's could either serve or slay.
Just think about it, Caesar was an acting figurehead of the Templars and Brutus was an Assassin. It would put a new perspective on history.