Gotham might be the DC Universe's most famous city, but it's not the place Nightwing calls home.
Ol' black and blue actually resides in Blüdhaven - Gotham's neighbouring city and one that's just as if not more corrupt than Batman's OG stomping ground. The character's stayed there for decades now, even becoming a police officer for a time, so it makes sense for it to be the locale of choice in the upcoming production.
Blüdhaven has already made the odd appearance in the Arrowverse, sans Nightwing, so it's not as if it's an untested location. Gotham might have a unique kind of draw to it, but it's not the be-all and end-all of Batman's world; there's an entire country out there waiting to be explored, and even though the city might just be a quick trip across the bridge, it's a whole different kind of beast altogether.