What made Cesar Chavez an effective leader?
I think you mean Cesar Chavez the American Labor leader.
Cesar Chavez was a migrant worker who was able speak to other migrant workers as well as to represent them. He received support from organized Labor and he became educated in the Labor movement and non violent protest as time went by. He and others were able to make the American public aware of the working conditions of the migrant laborers and motivate successful boycotts of brands of producers who they were trying to negotiate with. He has become a Labor and Latino icon since his death.
He had successes early in his career as a grassroots labor organizer but his later work had difficulties and his last boycott was unsuccessful. He often fasted to promote his political action and the resultant health problems likely led to his death.
His Leadership was was not always successful and would be worth closer examination.
He was likely successful in the early days because he was one of workers elevated to a spokesman level. He would have had some ability as a speaker. He established contacts with organized Labor which probably guided him in the early days. An intelligent, and dedicated representative with a motivated group of workers is a powerful force in a democratic country.