Just a guess, but maybe its to make diagnostics easier. If something is wrong they can image the cables and look for breaks.

There is not much in the way of 'elements' on Mars. If you look closely the cables are encased in something, but static electricity and dust storms are the largest problems and fairly easy to deal with.

For just a moment, I found myself worried about how they would hold up when it rained...

Wouldn't that be a nice problem to have. The only way to fix it would be to send some humans to the rover for a repair job...