A few years ago I made a serious attempt at understanding how creationists think, why they think it's so important to stick to this idea. It was difficult to ask the right questions, but the impression I got whas that they (those I talked to on the internet, they were all Swedish) didn't really think that it was a very important question at all, but they found the idea of direct creation more attractive than the idea of indirect creation and evolution. It's probably just the internal culture in their churches, how they are used to talk about things.
I find this annoying, since to me it seems that they just decide not to think about how we learn and understand things about nature and therefore they sort of miss the point. I don't think they are any less rational than any of us, only much less empirically inclined. Reason seems to be important to them, but investigating and evaluating the physical world is not. (Remember, this is my interpretation of the creationists I've been talking to, I'm sure there are all kinds of them.) You can do all sorts of reasoning and be extremely cerebral without going out to see the evidence, and then you also end up with all sorts of conclusions which are not necessarily related to what the world really is like.
As you might have noticed, the discussions about the credibility of evolution now ended up to be about the role of science in society -- as we can also see from the incredibly infected (and stupid) debate in the US. I can not come to any other conclusion that this whole thing hurts both sides, research and religion, by focussing the attention on the wrong things. I recently noted that some churches have initiated an "evolution weekend" next year. This is probably a good thing.
In the end, the best answer I've found to creationists is still the filk song Word of God by Catherine Faber (follow the link for the complete lyrics):
Odd, long-vanished creatures and their tracks & shells are found;
Where truth has left its sketches on the slate below the ground.
The patient stone can speak, if we but listen when it talks.
Humans wrote the Bible; God wrote the rocks.
It can be downloaded from The Virtual Filksing, performed by Kathy Mar. (I have the CD. There is also a beautiful neopagan song about fire, water and smoke -- this always makes me smile and think about the candles, holy water and incense generously used in a church (Lutheran!) I know in Uppsala.)
By the way, if you are interested it might be worth taking a look at what Dr. James F. McGrath has to say on the topic of creationism. Or about intelligent design:
An Immoral Godless Pseudoscience.