As you're no doubt aware, "tax" is a dirty word in the current U.S. political climate, certainly much more so than "subsidy". I suspect that goes a long way to

As you're no doubt aware, "tax" is a dirty word in the current U.S. political climate, certainly much more so than "subsidy". I suspect that goes a long way towards explaining why we have a number of different subsidies related to renewable energy but not a single carbon tax.