Why? Dick Stevenson, an editor at the New York Times, writes:

Obviously we were aware of the reports that Biden had likened the Republicans to terrorists. But we had no first-hand (or even second-hand) confirmation, and the vice president’s office was disputing that he had said any such thing. We debated whether we needed at least to take account of the controversy, but decided against doing so since we could not establish that Biden had said what was being attributed to him. Maybe there is more to this than we know. But on the face of it, it is a classic example of how what were once pretty clear-cut decisions based on well-established standards are now complicated by the reality that stories increasingly get injected into the public dialogue quickly and often with minimal journalistic vetting — leaving news organizations at risk of being perceived as deliberately ignoring them if they make a judgment against publishing.