The Bush Administration had to know that it would be political suicide to knowingly lie to the American public about Iraq's weapons of mass destruction.
I believe that they were certain, regardless of what doubts may have existed, that weapons would be found. I believe this is why they pursued this avenue of justification.
The truth is that people like Paul Wolfowitz and Donald Rumsfeld had been pushing to oust Hussein during the Clinton Administration but Clinton would not heed their wishes.
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Some will argue that the desire to invade Iraq was an exercise in taking over a weakened Middle East nation to exert stronger American influence over Arab issues and oil supplies. The letter claims it was to mainain the safety of American citizens from the use of WMDs. That is an issue for a whole other debate.
My opinion is that, after 9/11, they had the justification they felt they needed. It did not matter whether or not Hussein was involved in the attacks, they knew the connection could be made in the minds of Americans, so they set out making that connection. They never came out and said that Hussein was responsible, but they uttered his name in connection with Al-Qaeda often enough that many began to believe the connection.
The use of WMD as a justification seemed obvious to them. Iraq had used chemical weapons in the past and Hussein would have liked nothing more than to get his hands on nuclear technology.
They were certain that Hussein had stockpiles hidden where they had not previously searched. The administration was careful to never come out and say they had definitive proof. This allows them to now claim that it was faulty intelligence or to use semantics to distance themselves from earlier claims.
Either way, I echo what pan6467 says, we have a right to know the full truth and the Bush administration has an obligation to the American people to divulge all information. Do we forget that our system of government demands that our leaders are answerable to its citizens?