Originally Posted by
Badou
And another problem has been direction. He's the Robin that grew up, but unfortunately outside of that no one has figured out what to do with Nightwing's character. He's too old to be in the Teen Titans now and he will never be in the Justice League because Batman is there. He also isn't Batman's sidekick anymore so he is just another solo hero in a medium filled with solo heroes. Now he is competing against other characters, like Spider-man and Daredevil, that do what Nightwing wants to do but far better. He's been a circus performer, a student, a cop, a museum curator, a male model, a bartender, owner of Haly's circus, worker for Wayne Industries, and a spy. He's never found a solid direction the way Daredevil is a lawyer, Spider-man is a student who wants to become a scientist that funds himself by being a photographer, or how Superman is a reporter. They are constantly trying things hoping something eventually works to start building from.
There are probably other factors but these to me feel like the major ones that have really made it difficult for Nightwing to catch on and grow into something that people would want to use more outside of comics.