If the grown titans can't equal or surpass the league in importance in events, facing foes and be impactful as a whole. Then there is no point to the characters existing. Their growing is done. They need to play as a big guys now. I don't think dc views them as such a team.So they restrict them. As said, Rebirth manhattan plotline was never taken forward. It was introduced in titans and then discarded. The best issue in titans was clark and wally meeting. Neither wally nor the titan had anything in doomsday clock. As long as their skewed view is like that. Titans will be floundering as bgrade copies of justice league who is just bteam. That is neither the charcters faults nor the teams.As a team titans have better dynamics. They can give progression and can even accommodate new dynamics with new characters.