As for Chalabi, and any other liars our neoconservative friends were taken in by, I have to believe they let themelves be taken in so easily because they wanted to be taken in. They were looking for an excuse to go into Iraq and jumped anything without thoroughly vetting the source.
As much as some want to pile on "intelligence failures," the CIA was sceptical of Chalabi and his claims but their cautions were ignored by the necon axis around Cheny and Rumsfeld.