Well, why should you believe in evolution? Because it helps explain reality better.
But what if you didn't understand evolution at all? Then it wouldn't help you explain reality better. This is how most Christians avoid believing in it (of the ones who don't). Their understanding is so poor that it does not seem any better an explanation than God.
They reveal this when they say things that equate evolution with man being created by chance, or when they say atheists think time, in large enough quantities, can do anything. Or when they say that lions and tigers can't make fertile offspring (ie that species exist) and think that proves their point. Or when you point out that two *slightly different* lions can have fertile offspring, and they think that has nothing to do with evolution.
Notice how under this explanation, creationists are not wicked or even horribly flawed, just a bit ignorant. And ignorant of something that won't really help them much in their lives anyway.