Here's a hypothesis: denial is largely a product of the way normal people think. Most denialists are simply ordinary people doing what they believe is right. If this seems discouraging, take heart. There are good reasons for thinking that denialism can be tackled by condemning it a little less and understanding it a little more.Amongst the rest, Jim Giles points out the infectious qualities of a lie when presented to such an audience and Richard Littlemore writes about the corporate interests that benefit from denial.
Whatever they are denying, denial movements have much in common with one another, not least the use of similar tactics (see "How to be a denialist"). All set themselves up as courageous underdogs fighting a corrupt elite engaged in a conspiracy to suppress the truth or foist a malicious lie on ordinary people. This conspiracy is usually claimed to be promoting a sinister agenda: the nanny state, takeover of the world economy, government power over individuals, financial gain, atheism.
At the heart of the phenomenon is the breakdown of trust, "suspicious thinking", something that is more pervasive than you might think. It is at the heart of the urbane cynicism, which is now a media requirement for interviewing politicians, something that has stifled any attempt at honest expression of anything other than the approved line. It has led the assault on public sector workers accused of being only concerned with 'producer interests', rather than the welfare of their 'customers', leading to a battery of targets and performance indicators as a way of controlling the brutes. It is also central to the idea of politics as nothing more than a vehicle for naked self-interest.
None of this is to deny the importance of evidence-based scepticism, but that requires work, research and the willingness to admit that you have been talking complete bollocks for the past twenty years (in my case actually teaching it) without too much embarrassment. That isn't easy. And sometimes we do have to take things on trust and, on many occasions, we will be right to do so.