I have written enough on how much superior carbon taxes are to cap-and-trade as a CO2 reduction methodology (if we really are going to do "something," which I hope we don't). An index of these articles is here.
In the title, I say "another" reason, becuase the number one reason we won't see a carbon tax is that politicians greatly prefer an indirect tax over a direct one, even if it is far more inefficient. This was explained directly and clearly to me by the author of California's cap-and-trade program.
Close behind this, in second place, is the fact that cap-and-trade spawns a dizzying array of lobbying and special interest influence possibilities that carbon taxes do not, and all those lobbyists mean more power and campaign contributions for politicians.
But here is another reason why it will never happen: Too many very influential Democrats have substantial investments in start-up companies whose entire existance depends on living in the cracks of cap-and-trade, particularly in generating various dubious offset schemes. Al Gore is the most obvious example, but apparently Obama's new climate czar Carol Browner sits on boards of such companies as well.