A weightlifting bar is built for the Olympic lifts. It spins more — quality bearings or bushings let the plates rotate freely, which takes strain off your wrists and forearms in the clean and snatch. It also has more whip, a controlled flex that helps the bar move with you through the lift.
A powerlifting bar is deliberately stiff. In the squat, bench, and deadlift you want the bar to stay put under maximum load, and less flex means more stability. The aggressive knurling locks the bar into your hands — on a heavy attempt that’s a safety feature, not a comfort one.