Why the Knicks do it
Per Marc Berman, the Knicks are not moving Morris unless for a "Star-like" player. Debating how bright isn't important. He helps space the floor, solid defender, efficient scorer who should open up the floor for the Knicks assuming they view RJ and Randle as the primary offensive pieces.
They add the following players to their roster:
Kelly Oubre Jr. ($15.6M, 2yrs remaining)
Why the Suns do it
The Suns have a very solid starting group, but the bench hasn't met a similar standard. Trier being the sweetener there can outright score with the best, and can really take a hold of the bench rotation and solidify scoring. To be fair, the Suns are more than likely NOT dealing Oubre, but incase they are, this could help round out their rotations to make a run for the post season.
They add the following players to their roster:
Marcus Morris ($15.0M, 1yrs remaining)
Allonzo Trier ($3.5M, 2yrs remaining)