Because during daytime the temperature will increase which causes the particles in the cable wire move randomly. This causes the cable to expand. During night time the temperature in the surrounding will be lower which cause the particles in the wire cable will not move randomly/arranged compactly. This cause the cable to be contract during night time. So when the cable wire is fixed tightly to the poles,during night time it will contracts and there is a chance the wire cable to break off

Sorry I don't know bmr