Plato wanted to develop in the people a passion for those values because they are real and eternal and never changing. Therefore, they could give a unity of purpose and meaning to Athenian life - a reason for living and participating. Isocrates realized that his relativistic value system did not have the psychological force to draw people together into the common bond of unity and fraternity that would stabilize society. Only through education. Isocrates, like many modern thinkers, tends to view education as the savior of the world.
Human nature is distinctive because of man's ability to communicate, to speak.
It appears as though Isocrates may see this as a quantitative, rather than qualitative difference. Education should teach people to speak. Speech distinguishes us from animals. Speech was the key to political power. Speech integrates many aspects of human ability.