The reasons are not listed in any particular order.
1. Anti-science ideology. First, the scientists displaced earth from the center of the universe; then they displaced god from the story of human creation. Now they claim, based on nothing more than mountains of scientific evidence, that human activities, mainly involving the burning of fossil fuels, are altering the earth's climate in ways that are dangerous for humans and the ecosystems in which we live. The anti-science ideologues, who are mainly to the right ideologically (although, ironically, they receive some aid and comfort from the emergence on the left of "post-normal science"), are a menace not only on specific issues such as climate change but to the general project of enlightenment, which has been ongoing since the 17th century and has contributed almost immeasurably to the material improvement of human life on earth.
2. Educational impoverishment. This is as much a source of the first reason as it is a reason in itself. The vast majority of Americans are simply incapable of understanding even the simplest aspects of climate science, let alone its messy and uncertain implications for social welfare, without the intermediation of the popular media. The problem there is that individuals increasingly are tuning in ideologically-biased news sources that are more interested in indoctrinating and reinforcing than educating the views of their customers (which Richard Posner, in his book Public Intellectuals, refers to as "solidarity value").
3. Lack of political leadership. Because climate change is (relatively) difficult for people to understand, especially given the more or less willful ignorance of a large part of the electorate, the issue requires an exceptionally high level of political leadership, which has been missing. In his first term, President Obama saved most of his political capital for the fight over healthcare (certainly an important issue, however one comes down on it), which left him more or less bereft when it came time to push for climate legislation in the Senate, after it had already passed, thanks (I can't believe I'm saying this) to the strong leadership of Nancy Pelosi, in the House. Throughout the 2012 presidential campaign, climate policy remained on the back-burner; no one seem inclined to talk about it. And with the current, politically-invented crisis over the so-called "fiscal cliff," it seems unlikely that climate policy will move to the front burner any time soon. One caveat: the Obama Administration has at least allowed EPA to move ahead with greenhouse gas regulation under the Clean Air Act (which is hardly an ideal vehicle for controlling emissions for reasons I've posted on earlier, see, e.g., here). Those regulations are currently in the hands of the DC Circuit Court of Appeals. However the judicial review concludes, it could redirect pressure back to the legislative process, where the President's leadership skills will once again be put to the test.
4. Fossil-fuel money. When it comes to politics, I've always maintained that votes are more valuable than dollars, which explains for example why groups with the most money do not always prevail on policy issues before Congress. For members of Congress, money is just a (very important) means of gaining votes for reelection, which is the ultimate end. In any conflict between votes and money, votes usually will prevail. But those are rare cases, so money may often determine votes. And when it comes to money, few groups have more of it than the fossil-fuel industry, which has contributed millions, if not billions, of dollars funding studies by climate skeptics and deniers (which sometimes go horribly awry, see, e.g., here) and lobbying Congress, several members of which appear to treat the industry as their primary constituency (perhaps along with the NRA).
5. Climate scientists are a crappy lobby. First of all, they have no money. Second, and consequently, they have no influence. Unfortunately, there is no well-heeled and powerful interest group with a large material stake in supporting strong climate policy. There is, of course, a broader "public interest" in a stable climate, but we can hardly expect that to drive policy.
6. Mismatched Time Horizons. Politicians operate on 2-, 4-, or 6-year election cycles; the climate operates on time frames (decades or more) that are far longer. This fact really should not impede the enactment of (at least ineffectual) climate solutions. Politicians are usually adept at making difficult decisions that only take effect long after the date of enactment (placing the onus on future generations of politicians to amend or repeal their efforts). Typically, however, politicians will enact such forward-operating policies only when they can internalize immediate political benefits while externalizing future political costs. And given the all the problems noted earlier (from anti-scientist ideology to disparities in lobbying prowess of interested groups), the current generation of politicians fail to see any clear present political benefits of present legislative action on climate change.
7. Anti-internationalism. At a time when the Senate cannot even ratify an international treaty that simply reflects the existing state of US law and contains no enforcement mechanisms of any kind because of claimed concerns with eroding US sovereignty (see here), it's not really surprising that US politicians might reject out of hand any issue that is high on the international agenda, especially one like climate change that is strongly supported by a (gasp) United Nations process. Don't look for any kind of rationality here; it is simply pathological.
Any other reasons?