I suspect they refuse to debate rationally because they already know in their hearts that ID raises a credible and substantial question, the answer to which would be fatal to an essential part of their dogma.
The last thing a materialist wants to hear is a theory and evidence to support it that there is a mind, an intelligence "out there" that is responsible for informing life in all its dazzling complexity, wonder, and beauty. When that evidence is grudgingly admitted by a dogmatic materialist, there is no need for anyone to invoke religion. The materialist will invoke it himself, hestitatingly and fearfully.
ID raises a lot of questions, just none that can be addressed by evidence.
If you know of an ID assertion that can actually be tested, let us know about it. Perhaps you can discuss the ID research program. Perhaps you can name an irreducibly complex biological structure, one that would change your mind if it turns out to be made of separately functional components. Too bad the flagellum turned out not to be that component.