Laziness. Joe Frazier started this. They'll throw the hook by pushing off the front foot, generating the force by pushing. You can push pretty hard that way, but it is still a push. It also screws you over when you want to throw a right hand behind the hook.
If you throw the hook by turning your weight over the right leg, then using the right side of the body to whip the punch, you'll get more snap. And you'll be in a position to throw a right hand.
Always turn your weight in front of your punches.