The team isn't the kind that Nightwing would be on.
This is meant to be a team of the most dangerous individuals of the DCU coming together. Martian Manhunter, Hawkman, Green Arrow, and Catwoman aren't afraid to do what is necessary to get the job done. Nightwing follows the "one rule" of Batman.
It's not a snub, it's just that he doesn't fit the profile of the team.