why does it take a great deal of courage for Kino to make the decision to go to the city?

He is moving outside of his comfort zone and actually challenging the way things have always been. Further, Kino is, in many ways, innocent to the "ways of the world" and will be shocked when he sees how the city operates. He has little feel for the evil that exists in the world and the way in which the people in the city will try to take advantage of him.