The reasons range from economic to religious. The economic reasons are obvious: Exxon Mobil does not want to become a footnote in the history of mankind. They want to dominate the economic landscape as they do now. Coal interests are similar. Automobile manufacturers, paper mills, and thousands of industries are affected. Politicians owned by these are easily understood; they are nothing but alternative means for expressing the greed of companies and institutions the activities of which have precipitated and accelerate anthropogenic global warming.
As for the common man who is wholeheartedly into the denial, his economic interest is to keep a system in place that provides him a job and his children jobs. The ironies begin to emerge when we bring up children, though. You should not be surprised that apocalypic views of human history are part of the denial process, part of the deferral of responsibility, accepting the idea that no matter what there will be an apocalypse and so nothing really matters.
In Genesis in the Old Testament you will find that Man has been given dominion over the animals and over the land. This is the root base of the religious view of anthropogenic global warming. One form of the denial is that God would not allow mankind to exercise his dominion to a disadvantage of the planet. Another form, the more prevalent, is that He is large, but we are small, so our activities will count for little on a planetary scale. In fact the psychology of this has been reduced to slogans that accuse those who face facts and accept responsibility of hubris! That they are so full of themselves that they believe individual activities have a global effect!
One of the things we know about human beings is that they do not really understand numbers. A billion people is fundamentally incomprehensible to the average man or woman. Actually, a thousand is about as far as most imaginations reach for making tangible an abstract number. So when there are 7 billion people on the planet and innumerable cattle munching way producing belches (yes, it happens more at that end of the cow than the other) of methane (which is 4 times as effective as a green house gas), the equation is too complicated for most people and they give up. More than that, they succumb to the idea that things must in fact be simpler, so complicated things must be wrong.
The politics of all of this is interesting. Obama playing the Middle Game is automatically subscribing to half or more of the irrational ideas people have. It is clearly a time for someone with spine to stand up and lead, but that person has not yet emerged ... much to our own discredit. To lead means to take an unaccustomed path and to make changes by sheer force of will that cannot be easily undone. That's what we need. Instead we get sleasy lawyers playing the OJ game with the public imagination!