Yeah, Laika, it's the same reason Richard Dawkins (a professor of biology) refuses to debate uneducated Xian apologetists like Ray Comfort: it looks good on their resume, and by giving them credibility by creating a false equivalency, but does little for the scientist's cred...
Matt Dilahunty was talking about this debate a few weeks back on the Atheist Experience Show, and he and Martin had similar reservations about how it's a 'win' for Ken Ham, right off the bat (an opportunity for Ken Ham to pull off a Gish Gallop):
http://www.youtube.com/watch?v=D_2S3JMM19Q
(The comments on Bill Nye debate start at the 47:00 mark)