A carbon tax would force additional costs on energy companies, who are already scrambling to lay people off in the face of low oil prices. Some would probably be hired back into the renewables/green sector, but it would probably have the net affect of getting a substantial number of people laid off. No politician wants unemployment to increase in their voter base.

If enacted at the federal level, it would increase the price of all forms of energy. The average consumer, who sees the low price of oil as a small silver lining on a large post-recessionary cloud, would be loathe to pay more and would blame the government for trying to take away the one "advantage" they've recently received. No politician wants unhappy voters blaming them for higher gas prices and electricity bills.

Not to mention the enormous and protracted legal battle that big oil and coal companies would be sure to wage against a federal carbon tax.

The only thing that will get enough of the population concerned about unsustainable carbon emissions is when they see the things closest to them - food, water, housing, clothing - decline in quality or increase in price due clearly and demonstrably to carbon emissions. They need to believe that carbon emissions are the fundamental, undeniable cause of rising sea levels, a decimation of crops or livestock due to strange weather patterns, an increase in natural disasters, etc. Nobody cares about an intangible and "scientifically debatable" issue like global warming due to carbon emission until their house is underwater, especially when pollution is great for the economy in the short term.