Fashion. Your choices these days are mostly that or black anodizing which imitates carbon fiber like laminate flooring pretends to be wood, and unlike the real thing scratches are a different color from the surface.

Some parts need to be a certain size without much strength. Plastic (Carbon Reinforced Plastic, aka "Carbon Fiber" in the vernacular) is lightest followed by aluminum, titanium, and steel. That matters on paper and people care, although the practical impact may be insignificant - my carbon fiber brake blades save 21g over aluminum although that's only 0.02% faster up-hill saving one second on an hour long hill climb.

Manufacturing constraints. You can make aerodynamic wheel shapes you can't extrude and bend into a rim.

Manufacturing costs. A molded carbon crank is less expensive to make than a forged hollow aluminum crank.

Marketing that's convinced people to pay more for carbon fiber.

I'm not opposed to that look with carbon fiber cranks, brake levers, rear derailleur parts, and in my hub shells; although being honest that's what it comes down to.