Because they design their arguments specifically to sound good in such a limited debate setting, especially if their opponent is there to actually debate honestly. All they need is one line that makes you go "uh, um, well I'm not up to date on the specifics of that, but I would think that. . ." and suddenly science doesn't have all the answers. The person debating the merits of science represents "science" as a whole in the opposition's mind, not only their limited knowledge of it. So they design their arguments to be a series of "gotchas." which can lead even the best of debaters to attack the question instead of their nonsensical arguments, which they take as science not being able to stand against the mighty weight of the bible. And the audience cheers for the mighty savior, hallelujah.
In short, Creationism is about sounding scientific, without having to do all that... science... stuff. You just pick up a Bible, say "God did it" and *boom*, instant expert.