Passing Strange
Charles Krauthammer tries to explain it in the Washington Post - Liberal Democrats' Perverse Foreign Policy:

It was the left that led the opposition to war in Iraq. Now it is the left that is most strenuous in urging intervention in Liberia. Curious.

No blood for oil, it seems, but blood for Liberia. And let us not automatically assume that Liberia will be an immaculate intervention. Sure, we may get lucky and suffer no casualties. But Liberia has three warring parties, tons of guns and legions of desperate fighters. Yet pressure is inexorably building to send American troops to enforce a peace.

There are the usual suspects, Jesse Jackson and the New York Times, but the most unapologetic proponent of the no-Iraq/yes-Liberia school is Howard Dean, Democratic flavor of the month. "I opposed the war in Iraq because it was the wrong war at the wrong time," says Dean, but "military intervention in Liberia represents an appropriate use of American power."

Why?
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What is it that makes liberals such as Dean, preening their humanitarianism, so antiwar in Iraq and so pro-intervention in Liberia?

The same question could be asked of the Democratic Party, which in the 1990s opposed the Persian Gulf War but overwhelmingly supported humanitarian interventions in places such as Haiti and Kosovo.
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The only conclusion one can draw is that for liberal Democrats, America's strategic interests are not just an irrelevance, but also a deterrent to intervention. This is a perversity born of moral vanity. For liberals, foreign policy is social work.
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Hence the central axiom of left-liberal foreign policy: The use of American force is always wrong, unless deployed in a region of no strategic significance to the United States.

I'm not as high minded as Mr. Krauthammer. I just think they're nuts. As for the question at hand:

Regarding Liberia, it is rather odd for the Europeans, who rail against U.S. arrogance, to claim that all the armies of France and Germany, of Europe and Africa, are powerless in the face of Charles Taylor -- unless the Americans ride to the rescue.

Odd indeed.