Because if they knew, their "grand plan," whatever it is, would fall apart.
As far as we've been able to figure out their plan consists of us losing the war, Bush's utter failure, loss of American life, and the economy in the crapper.
And they ride to the rescue and seize power again.
Imagine.
Bingo. Democrats FEAR another American military victory on Bush's watch. He'll be that much more difficult to unseat in '04.
Sadly, if they return to power via our demise, they won't be able to rescue US from what they've brought down upon US.