Why the theory of Intelligent Design is so hard to rebut.

It is because there is no clear theory of ID. John Pieret of Thought in a Haystack posted an interesting bit recently. Well, he posted more than a bit, but here is the bare minimum that I that describes his point.  I, on the other hand, am posting only a bit, so you can follow the link for more.

The many interesting examples that dominate the ID discussion—the little tail on the bacterium, our eyes or our blood-clotting mechanism, the explosion of new life-forms in the Cambrian period—are just snapshots of things in nature. They are not "evidence" for anything and won't be until the ID theorists develop a theory of how their "designer" works. Once they provide a well-articulated version of their central claim, we can decide whether or not our eyes—or our tails— support their theory.

Darwin knew this: 

About thirty years ago there was much talk that geologists ought only to observe and not theorise; and I well remember some one saying that at this rate a man might as well go into a gravel-pit and count the pebbles and describe the colours. How odd it is that anyone should not see that all observation must be for or against some view if it is to be of any service!

Pieret is quoting Karl Giberson who is describing a debate he had with Stephen Meyer.