According to the book there could be several reasons. If the truth would take too long or cost too much to find out, smaller media outlets can't afford to pursue it; instead they rely on the larger outlets like AP or UPI or New York Times, so if those guys don't follow up, the smaller outlets won't either. If the truth is, or is feared to be, unpopular with their advertisers, it could get downplayed or stifled. If the truth is, or is assumed to be, not of interest to the viewing audience, it can get discarded. If the truth is, or is projected to be, a less lucrative story, they'll choose the less-accurate but more-profitable one.

None of these are good reasons but they're all perfectly plausible.

Media here is just as loony over swine flu. Thousands of people will die!!!! without mentioning that thousands of people die EVERY YEAR from regular flu. Silliness.