Because carbon is the religious original sin of our time. No matter what one does (carbon tax, cafe standards, other efficiency standards, etc) one will always be tarnished by committing the sin of using carbon and one must always be looked down on for it, at least to some people. It does not seem possible to have a reasonable conversation with these people along the lines of, ok set a carbon tax for the supposed externality releasing co2 causes and then stop wasting brain cycles on this. Let people choose under the new price regime and mind your own business because you don't know what their utility function looks like, what seems like waste to you is just the delta between your respective utility functions and you need to get along with each other for society to function, which necessarily means mind your own damn business once the harm that wasn't previously in the price is reasonably in the price.