The basic problem is that a carbon tax would be seen as a new tax. New taxes are typically unpopular. This makes it hard for politicians to support a carbon tax, as they are beholden to their constituents, and their likely desires to avoid such a tax. This in itself makes it unlikely a Carbon Tax would ever be implemented.  Further, a carbon tax would require complicated enforcement mechanisms. These mechanisms would impose an administrative burden on the state more severe than that created by a cap-and-trade system. In a carbon tax, emitters would pay a tax for every tonne of carbon emitted. This requires that the government know precisely how much carbon is being emitted by energy producers. This is not easy to determine, and requires that a government put in place monitoring mechanisms. Deploying these mechanisms universally would be very complicated, expensive, and require much administration. Then, ensuring that all these monitoring devices operate properly and that all energy producers comply with the tax would also involve a substantial administrative overhead.  This would be equally as complicated as a cap-and-trade system, which requires much the same administration, but also encourages other companies to keep tabs on their competitors and their emissions. Credit trading spreads the administrative costs of carbon taxation over a number of companies, all of whom will be acting to protect their carbon credit investments and the stability of the market they are traded on.