Science envy, mostly. Science has been remarkably effective in learning about the physical universe, and helping us get along it it. Creationists would dearly like to find a way to get some of that prestige for their religion. As you might have noticed, most Christians don't worry about it, finding faith in God sufficient.
And, as you might have noticed, when creationists do make testable scientific claims, it's pretty easy to debunk them. So it's good for Christianity to keep foolish and spurious apologetics from deluding those who might otherwise come to Christ. (yeah, I know you don't believe; it's O.K. with me, because I'm Christian, and I know that atheism won't automatically send you to Hell)
We're trying to avoid what St. Augustine warned us about, long, long ago:
"In matters that are so obscure and far beyond our vision, we may find in Holy Scripture passages which can be interpreted in very different ways without prejudice to the faith we have received. In such cases, we should not rush in headlong and so firmly take our stand on one side that, if further progress in the search for truth justly undermines this position, we too fall with it. We should not battle for our own interpretation but for the teaching of Holy Scripture. We should not wish to conform the meaning of Holy Scripture to our interpretation, but our interpretation to the meaning of Holy Scripture."
Putting a sharper point on it, he wrote:
"Often a non-Christian knows something about the earth, the heavens, and the other parts of the world, about the motions and orbits of the stars and even their size and relative positions, about the predictable eclipses of the sun and moon, the cycles of the years and seasons, about the kinds of animals, shrubs, stones, and so forth, and this knowledge he holds with certainty from reason and experience. Now it is a disgraceful and dangerous thing for an infidel to hear a Christian, presumably giving the meaning of Holy Scripture, talking nonsense on these topics, and we should take all means to prevent such an embarrassing situation, in which people show up vast ignorance in a Christian and laugh it to scorn... If they find a Christian mistaken in a field which they themselves know well and hear him maintaining his foolish opinions about our books, how are they going to believe our books in matters concerning the resurrection of the dead, the hope of eternal life, and the kingdom of heaven, when they think their pages are full of falsehoods on facts which they themselves have learnt from experience and the light of reason? Reckless and incompetent expounders of Holy Scripture bring untold trouble and sorrow on their wiser brethren, ... to defend their utterly foolish and obviously untrue statements, they will try to call on Holy Scripture, .. although they understand neither what they say nor the things about which they make assertion. "
St. Augustine, De Genesi ad litteram libri duodecim