Because the Bush administration manipulated pre-war intelligence against Iraq--they have no credibility when they make claims against Iran about nuclear weapons.
It's the boy who cried wolf (or, as my childhood friend used to say, "wuff").
It's not that the Iranians aren't a threat. They very well may be. We just don't know. And we're not going to be able to find out as long as Bush and Cheney are in office because they have no credibility on intelligence matters. After 6 years of lies (see Iraq) and incompetence (see Katrina) no one is listening to what they have to say.
We can't go two more years without accurate, credible, up to the minute, untouched-by-Cheney's-hands, solid information about Iran that the world can rely and act upon.
So for the sake of national security and to deal with the potential threats from Iran, North Korea, who knows where, we need to impeach Bush and Cheney immediately.