Why doesn’t asphalt stay that nice rich black colour?

The UV rays from the sun breakdown the carbon bonds in the asphalt oil thus causing the colour to change from black to gray. It is this oil that binds everything in the asphalt together so when it is depleted erosion starts. This oil is also what keeps the asphalt flexible. So when the asphalt is gray and oxidized it is less flexible and more prone to cracking.