Robert Samuelson in the WaPo offers a good explanation.
George Bush hasn't much discussed what could be his biggest economic problem. It's not budget deficits or jobs. It's the possible crash of the dollar on foreign exchange markets. Even if Bush understood it, he would be hard-pressed to explain it to the public. Worse, there are no obvious ways to prevent it. Nor is it certain how big the threat is. Little wonder Bush hasn't said much. If John Kerry had won, the situation would have been the same. But a dollar crash, if it occurred, could trigger a terrifying global slump.....Yikes.