Not to mention a steel tubeframe is about the simplest chassis you could make, and the current karts in that form are still way more expensive than they should be. The price of a carbon chassis would be obscene.
Plus, the strength and stiffness that CFRP can obtain, might not exactly be beneficial to a kart chassis. The peculiar nature of how a modern kart chassis works means that it needs to flex in order to perform properly. Think more in line of a carbon fiber suspension, rather than an ultra-stiff carbon tub chassis.
I think I recall someone making a carbon chassis as an experiment nevertheless. Not good for karting anyway to put that idea into production.