Because the mere mention of his name reminds them and everyone else that they were wrong about the invasion of Iraq, something most of them still deny and the rest try to forget. The few who admit they were mistaken usually claim that no blame attaches to their error because it was universal: everyone of any importance was wrong, so no one was wrong to be wrong. Conversely, if someone was right, that just proves that he wasn't someone of importance: why else would his objections at the time have been so easily ignored? Or it proves he secretly wanted Bush's foreign policy to fail. Hence the distasteful Schadenfreude when things fell apart.
So, in review, the only way to have been right about Iraq was either (1) to have been a no-one, beneath notice, or (2) to have been right for the wrong reasons (e.g. because of insufficient indignation at "Islamofascism" or latent anti-Semitism or cynical Realpolitik). It being impossible to deny that Senator Hagel was someone of importance when he opposed the war in Iraq, only one kind of explanation is left. The discredited must try to make him look dishonorable. This also explains why the Washington Post, a guardian of conventional wisdom (and in this case one of its parents), is desperately casting about for reasons to oppose Hagel's nomination.