Paul Krugman mostly nails it:

What is it about Mr. Gore that drives right-wingers insane?

Partly it’s a reaction to what happened in 2000, when the American people chose Mr. Gore but his opponent somehow ended up in the White House. Both the personality cult the right tried to build around President Bush and the often hysterical denigration of Mr. Gore were, I believe, largely motivated by the desire to expunge the stain of illegitimacy from the Bush administration.

And now that Mr. Bush has proved himself utterly the wrong man for the job — to be, in fact, the best president Al Qaeda’s recruiters could have hoped for — the symptoms of Gore derangement syndrome have grown even more extreme.

The worst thing about Mr. Gore, from the conservative point of view, is that he keeps being right. In 1992, George H. W. Bush mocked him as the “ozone man,” but three years later the scientists who discovered the threat to the ozone layer won the Nobel Prize in Chemistry. In 2002 he warned that if we invaded Iraq, “the resulting chaos could easily pose a far greater danger to the United States than we presently face from Saddam.” And so it has proved...

Which brings us to the biggest reason the right hates Mr. Gore: in his case the smear campaign has failed. He’s taken everything they could throw at him, and emerged more respected, and more credible, than ever. And it drives them crazy.

My only quibble is saying "his opponent somehow ended up in the White House."

Bush stole it. And if we don't repeat it every time the issue comes up, we become accessories to the crime.