Why Knicks do this?

NYK's are are trying to make a championship push. Karl Anthony Towns is a clear starter. Adding Vucevic gives them a starting caliber center that has played in nearly 95% of all games in his career. Vucevic is rarely injured. Availability is valuable. Vucevic off the bench would highly advantageous. Losing Robinson's defense may hurt, but when he's barely on the floor will it hurt that much. Precious Achiuwa is expendable as a salary filler as Sims can fill that 3rd center role.

Why bulls do it?

Bulls take a flyer on two young bigs in what is considered a rebuild/retool season. Mitchell Robinson has to prove he can stay healthy and Achiuwa can get big minutes on a team with limited big men. Knicks add two 2nd round picks because of Robinson's injury history.