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.
A great example of that is how he attempts to validate the flood story. Ham's argument about how we have multiple species in a 4,000 year old flood story is to summarize that using dogs as an example different types of dog species all came from the original same line. Similarly, cats et al.