Because the resin used in carbon will never exceed the hardness of alloy and as mnigro said, the torque will damage the resin and thus the carbon structure. Carbon structure is amazing in tension, but crap in compression/crush so no applications such as spindle w/ crushing or torque forces.
The thing w/ handle bar...if you limit your crushing force (the bolt torque spec) and make the carbon thick enough, there's no real torque involved and so still works.
Edit: I'm sure Mr. Magura will chime in on this shortly.
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