The excuse so far is that it would imply a “moral equivalence” between Bush and Saddam.
I think the real reason the offer was refused is that Bush is an idiot. He already has a difficult enough time with preprepared speeches by his handlers. If he actually had to answer the big questions personally people would realize that the Emperor is actually not wearing any clothes and this whole charade would end.
Bush is a coward. Plain and simple. He hides behind his position while issuing Presidential Edicts which can’t be questioned or debated. The whole world must follow him or ELSE. And he’ll be damned if he ever has to justify it in front of a national audience.