From car and driver magazine:
Some of the carbon-fiber panels (splitter, rocker extensions, roof) will actually look like carbon fiber, with a clear-coat finish over the weave. Chevy has a new ingredient for the finish that resists the ultraviolet breakdown that’s been a problem. We don’t know the chemical ingredient, or who makes it—Juechter won’t say—but we do know it costs $60,000 a gallon. It’s used sparingly.
I'm guessing most aftermarket parts don't have it which is why they yellow and delam after a few years in the sun. The cost would drive the price of the parts beyond what most people would consider reasonable.