I'd hazard a guess at the "obvious reasons:" Carbon's generally lighter than alloy (by volume, for example). Lighter = Weaker in a lot of people's minds. For something you crank down on literally, an alloy would bend where carbon would snap ..
NOW, how true is this?
Not very in today's world of material science/engineering/technology.
FWIW, Carbon can be "stronger" than alloys; i.e. can withstand more stress before failure. When said failure occurs, though, you will have different signatures .. maybe that's where the fear is ..
Largely, I think it's mental. People are still getting used to the idea of something as light as Carbon being stronger than alloys/metals. In case it's not obvious, most chain rings even on carbon cranks are made with alloys .. it's mostly always the crank arms that are Carbon.