Rain coat is usually made up of vinyl so that the water will not be absorbed by it. But if we make rain coat with cotton fabric, it will absorb more water and make the person wet. Rain coat is worn to escape the rain water but if we make the rain coat with cotton and then the rain coat will not be having any meaning. Instead we can wear our normal dress and get wet. Cotton allows the water to pass through it but vinyl will not allow water to pass through it which causes us not getting wet in rain.
Hope that might help you.........^_^