I'm not sure I can say Jack is a better leader than Ralph but he certainly is more effective on the boys than Ralph could ever hope to be. Ralph is not forceful by nature and could not manipulate like Jack could. Although Ralph is a good leader, Jack is more effective. Jack is naturally aggressive. Early in the book we see this through his repeated stabbings of trees. Later he learns to channel his aggression through rather clever manipulation. He learns to give and withhold when it benefits him (the giving of meat to Ralph). Jack learns that ruling by fear is much more effective than leading through consensus. Jack learns to rule by fear. The idea of a beast serves Jack because he claims he can protect the boys from their own wild imaginations. Ralph logic cannot compete with Jack's manipulation of the boys' primal needs and fears.