I agree with
@madguy30 that they've just kind of reached their respective peaks, more than likely.
But also, the main thing for Niang is that he doesn't provide defense, so if he isn't hitting shots (as in these playoffs), he's not very valuable. And even when he is hitting shots, in the playoffs, opponents work harder to hunt mismatches and expose defensive vulnerabilities, so he's more of a liability than the regular season.
For Morris, part of it is just that he probably isn't back to regular play after his injuries. He only got to play 33 games this season (only 27 with MN), so he might not be fully ready for the higher intensity of playoff basketball yet. Also, I kind of doubt he'll reach the level of playoff success he had at times with Denver. He and Jokic meshed together so well, in a way that I don't think he can with Edwards and KAT offensively.