Why Sunlight breaks down the polymer in a coax jacket over time, making it chalky, brittle and cracked. Once the jacket splits, rain and humidity wick into the braid and dielectric, which raises loss dramatically and corrodes the shield. That is why outdoor runs use UV-resistant (usually black, carbon-loaded) jacket material, and why any nicked or cracked section should be replaced.
Watch out Jacket losses are not the issue: the jacket carries no RF, so UV does not directly add loss. The damage matters only because it lets water into the parts that do carry signal.
UV cracks the jacket, water gets in, cable dies. Outdoor coax is black for a reason.