Because they lie. Routinely and often and deliberately. They said there were 100,000 people in mass graves in Kosovo. That was a lie. They said Iraqi soldiers were tossing babies out of incubators. That was a lie. They said Iraqi troops in 1991 were massing on the Saudi border. That was a lie. They said Saddam's attack on Kuwait was a total surprise. That was a lie. They said US troops had no combat role in Central America in the 1980s. That was a lie.
Even people who support the war have been less than impressed by Powell's supposed "Adlai moment" - Tacitus gave it good reviews for its strategery in driving a wedge in the antiwar camp, but seemed to be acknowledging it was preaching to the choir. Tacitus said that this NYT piece alleging ties between Iraq and Al-Qaida was more convincing than Powell (see his transcript in Word format), but such ties have been exhaustively analyzed, and the Bush administration previously admitted there was a weak-to-none case (until now once again it becomes politically expedient to assert the opposite. Same pattern of deceit and doublethink as always). Daily Kos has provided a large number of links and evidence that the FBI and CIA both reject any such link.
There isn't any link between Iraq and Al Qaeda. The main relevance is that if we go in, and do a poor job AFTER winning, that there might then indeed be a fertile ground for AQ to grow. Bush's Afghanistan reconstruction has been largely ineffective and insuffficient.