Originally Posted by
Digifiend
Gotham and Metropolis are grandfather clauses. It was just the thing to do back in the golden age - even Timely's books weren't all set in New York - that change at Marvel only happened early in Fantastic Four, they actually started in a fictional city which was then retconned to being New York. Obviously Barry Allen debuted before then (his debut being the commonly accepted start of the silver age), so Central City has the same grandfather clause as well. DC persisted with the fictional city thing through the 1970s, but then New Teen Titans in 1980 followed Marvel's lead, and the book was set in New York. Soon after, Green Arrow went for more realism, hence the switch from Star City to Seattle.
What's odd regarding Nightwing, is that his city, Bludhaven, was only introduced in the 1990s, so it doesn't have a grandfather clause. Wouldn't it have made more sense for him to be based in New York, where his Titans team had operated previously, or in Gotham, which was his original home as Robin? He's constantly having to use a Batplane or T-Jet to commute between two cities whenever he's working with the Titans! In fact in his latest issue, he complains about having to temporarily leave Bludhaven for Gotham for the Fear State storyline... only he's doing the same thing whenever he goes to Teen Titans Academy in New York.