Lets forget about the big business reasons to deny Global Warming caused by man for a moment.
One thing that doesn't seem to get a lot of attention is that most Global Warming denialists are also evolution denialists.
Yes, the connection has been made, but there hasn't been lots said, and motivations aren't really being looked at.
I'm going to hypothesize about the reasons.
Bible literalists love any sort of perceived ambiguity in science (even if it doesn't really exist). Why? Because they are brainwashed thinking that if anything in their bible is wrong, the whole bible is wrong. So the fact that scientists have not come up with a consensus on Global Warming, and some believe that it is mostly man made, while others believe it is mostly a natural climate change, while others think that it is a mixture of the two, and aren't sure about the magnitude of man made causes regarding the current heating of the climate.
It is the same tactic used by creationists who quote Gould regarding his hypothesis regarding evolution happening abruptly. The fact that there were and still are debates regarding this, makes creationists state that the jury is out on evolution.
Of course, the data shows the earth is heating up and that evolution happened. But because the EXACT mechanisms are figured out yet, gives creationists the ability to say that evolution and Global Warming is not real science.
Another reason could be that creationists don't think that there is such a thing as climate change, well, except when God makes it rain for 40 straight days. 45% of Americans believe the earth is less than 10,000 years old. So using logic, most of them don't believe there have been Ice Ages, since the last one started around 20,000 years ago, at least 10,000 years before God created everything.
Personally, I don't think that these people really think about such things as the real age of the earth. The majority just drool at the mouth when they perceive they have any chance that there is a science conspiracy.