Cooper took advantage of Ali pulling straight back from punches; frazier took advantage of poor punching habits. In the case it was how Ali dropped his hand to throw an uppercut.
I read that the force of Henry Cooper's left hook, after traveling 5 inches, was the same as a 16 pound bowling ball striking the headpin. Frazier threw his left hook off his left leg, which certainly is effective (especially for shorter fighters), but I don't think that it is the best way to do it. I think that his hook is considered 'on another level' is because he threw it so many times against Ali; while definitely an effective weapon, I don't think he had the best hook ever nor is he particularly close to the top.