Why do people laugh at creationists?
It's got to be established what the target audience is of any text. There's no point in writing War and Peace for young children likewise there's no point in addressing the problems of creationism to people who will never listen. AronRa on youtube makes this point clear, those professional creationists will not be swayed no matter what. Instead he like many others aims at the middle-ground, at those who simply do not know better or have been misled. So while the styles of Thunderf00t, DonExodus2, Pharyngula or any of the other big names on Web 2.0 will not stop those dyed-in-the-wool creationists, I would contend they provide a service to those who are susceptible to evidence.
As Thunderf00t says on each video: "why do people laugh at creationists? Only creationists don't understand why." Maybe this statement is wrong, evidence would suggest that the majority don't understand why creationism is mocked. Indeed where the majority thinks it appropriate to teach both sides when there simply aren't two sides to it. This is really a failure of the scientific institution to explain why there aren't two sides to this, or at least explain it to the layman. Those in the know understand the methodology of science, those not will still say as Aaron points out:
"It's just a theory"
The expectations of having someone just give up their belief on just one stimulus is unrealistic. If someone has had decades of cultural indoctrination and years of personal rationalisation, just simply showing them the evidence is not going to recant on what they hold to be true. It would be like trying to reverse the direction a river flows by blowing into it with a straw. The enormity of the task is not to be understated, if there is such a tactic that works I would be glad to hear it.
Fringe versus the mainstream
Aaron makes an excellent point about holocaust denial if it were a different audience like one in Iran. How do we take the views of the intellectual establishment and compete them against the views of the ignorant majority? In the USA, the figure who believe evolution without any intervention from God is around 14%, where in science the figure is around 99%. Why is there such a disparity between the two?
As for Scientology, I would contend that Scientology's fringe status has less to do with mockery and more to do with an ultra-competitive market. This is actually testable, put a religion into an open society and see how it thrives. The New Age movement is taking off in post-Soviet Russia, the memes are working in an open environment. Scientology is it's own worst enemy, being secretive in a free society is cause for distrust.
The contention as I see it is thus: the establishments that are selling history or science are not doing a very good job competing against religion. In the case of holocaust denial, it's no surprise that it's huge in countries that want to wipe Israel off the map. Likewise the biggest threats to evolution come from communities where there is Fundamentalist Christianity, the flock rallies around the preachers of ignorance. Is it up to the historical community to stand up in Iran and show the evidence of the holocaust? Is it up to the scientific community to show the evidence for evolution (and indeed the old earth / vast universe)?
The 'I' word
In the end I can truly appreciate why people like Dawkins see religion as corrosive, and why they are attacking it with such vigour. It's influence is part of the problem, but really it's the ignorance of the general population. For you and me, it doesn't matter one bit whether the holocaust happened or not. I could be oblivious to all the facts, all the evidence for the holocaust, and still live a perfectly functional life. Without any knowledge on history I would be susceptible to influence by those who appear to have knowledge on the topic.
When a preacher gets up and talks about evolution being wrong, that's what he's becoming. He's a beacon of knowledge for the ignorant. Now there's a chance that any of us are simply that, it could be that everything I have written in my blog is down to misinformation and I could be showing falsehoods. To the average person would it just be my word against another's? Like the conservapedia example, to the casual observer it would seem like scientists are scared of debating creationists. Most the population have no idea how a microwave works, or even how they get electricity into their homes to power it. It's ignorance that is the real enemy.
One final thought, there's been one recent movement that has gained weight despite it's absurdity: the 9/11 truth movement. It's something we can study both from a sociological and psychological perspective to understand just how these delusions of reality can spread with apparent plausibility.