Several reasons, starting with the principled reasons for opposing AGW which center on property rights.
First, what's a "fair" level of taxation? We can't pin down the damage carbon causes, therefore we can't pin down a reasonable level of taxation from the property rights justification. Common law for property rights holds that you're responsible for the damage you cause - how much damage does a ton of carbon dioxide do?
Second, what is it to be spent on? Many of the proponents of carbon tax advocate that the tax be revenue neutral, and the proceeds distributed per capita - while this falls in line with the compromise I'm willing to make with the welfare state, it doesn't bear the same justification (aligning the spending incentives of the public). From a property rights perspective, this fails - each person isn't affected equally, and some will benefit from any warming which does occur. (Should those who benefit from AGW be taxed, as well?)
Third, and perhaps most importantly, who the fuck are we "saving the world" -for-? The people who are going to be most negatively impacted by any global warming are those who have the greatest incentive to take advantage of the cheapest energy available - the poor, and carbon fuels.
If you want to fight the use of carbon fuels, there is precisely one way of doing it - coming up with viable alternatives. Not for the upper-middle class yuppies to use to microwave their lattes, but the gross amounts of cheap energy necessary to fuel a burgeoning third-world economy.
I think this has some potential, although the people behind it are on the opposing side of the political wall from me, and are too focused on the "yuppie."