The official reason given by the company is that carbon fiber allowed the submersibles to be positive buoyant. This in turn allowed them to use electromagnets t

The official reason given by the company is that carbon fiber allowed the submersibles to be positive buoyant. This in turn allowed them to use electromagnets to hold onto ballast, and in the case of a failure the submersibles would loose its weight and go back to the surface. It was also considered cheaper and smaller than using steel in combination with syntactic foam to achieve the same result.