We'd all like to think people are fundamentally rational. After all, people aren't paying priests to pray over their broken-down cars, they take those to the mechanic. But for the religious, and specifically for the creationist, the mindset for obtaining knowledge about the world around them is completely different. In the creationist mind, the rules that govern what will fix a car are not the same rules used to determine where the universe, the earth, and life came from. Cold rationality takes a backseat, and it only used when the results don't contradict the primary rule set. What is this rule set?
The rule set is Gnosis--revealed, divine knowledge. Many creationists are trained from birth to accept the words of a pastor, priest or other religious authority as unquestionably true. Even if some sects allow for errancy in their teachings, error is generally not assumed to the same degree as in Skepticism.
Attacking Darwin and his book The Origin of Species when challenging evolution seems foolish to most non-believers. After all, great progress has been made since those days, and the most convincing argument for evolution is found, not in Origin, but in the field of genetics. Evolution owes more to Watson and Crick than it does to Darwin, yet Darwin is the popular scapegoat. Why?
Because creationists have no concept of progressive knowledge gain due to an increase in the amount of available information. They have no concept of this because it is not part of their Gnosis. They are used to thinking in terms of unchanging, unyielding declarations from infallible religious authorities. So when they attack Darwin, they're not necessarily being deliberately incredulous, they're demonstrating the fact that they are unable to fathom thinking in any other way. To them, Darwin is our prophet, and Origins our scripture. When debating amongst themselves, the credibility of their prophets is of supreme importance. If they can undermine the prophet, they can undermine the message. So it's of no surprise they'd use this same tactic against evolution. They think of it in terms of just another religion, just another prophet to discredit. They don't understand that evolution doesn't sink or swim on the reputation of Darwin in the same way that Christianity does on the reputation of Christ.