Because in displacement, the feelings are transferred to a neutral person (i.e. yelling at your wife when you come home because your boss yelled at you at work) or an object (i.e. hitting a wall instead of hitting your wife).
But in sublimation you use your anger or frustration to your advantage ( i.e. in this case the person did the hardest work out in gym like burning more calories, lifting more weights).
I hope this helps, can someone also verify this!!!