Dick recently moved to Bludhaven in the DC Rebirth continuity, so it only seems appropriate that the DCEU’s Nightwing fight crime in that city rather than stick around Gotham City.
However, for the purposes of the Nightwing movie, it’s important that Dick has some distance from his old stomping grounds.
Nightwing still works with Batman and the rest of the Bat-Family frequently, meaning he spends a fair amount of time in classic Gotham City.
In 1996, Dick moved to Bludhaven to investigate a series of murders, later becoming the city’s official protector.