Assassin's Creed III will be in set colonial-era New York and Boston, with player traversing the frontier between them. New York and Boston are both great, but historically speaking, the city of Philadelphia, being the site of both Continental Congresses, is where I'd personally imagine the center of the secret clash between Templars and Assassins took place. Why it wasn't one of the main backdrops of the ACIII was one of my biggest questions.
During an interview with Joystiq, creative director Alex Hutchinson explained that Philly was ultimately dropped from the world map for the sake of gameplay. The team considered setting a section of the game in Philly, but realized upon closer inspection that the city's design wouldn't make a great playground for an Assassin.
"When we started out, Philly was one of the cities that we wanted to do. In the end, we decided against it, 'cause when we mocked it up... It's an architecturally designed city, so it's on a grid. Which at first we thought would be cool – the first city in AC to be designed on a grid – and then you realize ..." Hutchinson said.
His colleague picked up after him: "Well it's on a grid and it's flat, and the avenues are super large. So that's like three things that made us say it doesn't work in an open world."
Though players won't be able to travel to Philly on their own in ACIII, Hutchinson did say that "key events" take place in Philly, so maybe the city will get its due, after all.