Timothy Noah has a plausible theory about why this particular Bush lie has caught so much attention which he explains in this piece in Slate. (Part I is here).

The yellowcake lie landed on Page One solely because it occasioned a brief and fatal departure from the Bush White House's press strategy of stonewalling. "Bush Claim on Iraq Had Flawed Origin, White House Says" read a New York Times headline on July 8. Glancing through the story, Chatterbox initially puzzled over its Page One placement. Didn't we know already that Bush's yellowcake line was a lie? Then Chatterbox realized that the novelty component wasn't the lie, but the Bush administration's admission that it had told a lie. In the Bush White House, this simply isn't done.

[...]

The ugly reality about stonewalling and lying is that, if pursued with the proper discipline, it can be an effective public-relations tool. Mainstream reporters may contrast what a White House press spokesman says with what somebody else says, but they usually hesitate to state bluntly that Person A is lying and Person B is telling the truth. (An admirable exception is Dana Milbank of the Washington Post, who has devoted considerable energy to documenting Bush's falsehoods.) If a press secretary states consistently that up is down, most reporters will present this as a matter of opinion. But if he states repeatedly that up is down, then says that up is up, and then resumes saying that up is down, reporters will seize on the inconsistency and cry foul. Unlike disagreement between one person and another (or even disagreement between one person and the rest of humanity), a single person's saying one thing and then saying another is usually taken (sometimes unfairly) as prime facie evidence that a lie has been told.

Reporters, of course, are in the business of "objectively" presenting what politician say. It is well beyond their scope to evaluate what is said. This is the central paradox of "objective" reporting: as soon as one starts to deal with the matter itself, and not what is said about it or depicted, one enters the realm of "opinion" or editorialization. Opinion can be objectively reported, but objective evaluation can only be opinion. Confused yet?

(I've written about the way 'objectivity' is conceived in American journalism and the problems it raises in a previous post.)