Why was nonviolent action so effective, and what unique leverage does nonviolent resistance have in the face of power and force?

By using nonviolent tactics, one can bring about change and questions in the section of the population, not affected by the problem, but not strongly on one side or another. When nonviolent actions is met with a violent response it tends to cause others to view the nonviolent participants in a more sympathetic and favorable light, while making the more violent participants out to be "monsters".