Speaking as a Nightwing fan, I think it comes from remembering he's Dick Grayson, the first and most iconic Robin, who is their fourth-most represented character in general media outside the trinity. I would argue more people know about Superman, Wonder Woman, Batman and Robin than any of the rest of the Justice League. Moreover, it's well-established that Didio has gunned for him in the past and he's not the most beloved figure in the DC offices. It can become easy to feel he's getting shafted when someone's constantly got a gun pointed at his head... But even so, we're very lucky with respect to other characters. I love a ton of B, C and D-listers and Nightwing is always getting stories told about him. He even recently got an Elseworlds title! I don't like Damian, but he's very popular and even he doesn't get a solo.