Hard to move with your weight too far forward. Do you remember Ike Quartey? If you came topwards him, like Vince Philips did, he threw really nice combinations...But his thing was his left jab- lots of power on it because he got his weight way up on his left foot. Kind of a problem because, from there, he couldn't throw a right hand. Also his opponents could move away from him because, to follow, he had to rock his weight back.
Which I guess has nothing at all to do with throwing a left hook...