This Newsweek article probably explains much about why Bush insisted that Iraq had weapons of mass destruction. If what he believes to be true is contradicted by intelligence experts then he ignores them:
That NIE, made public Dec. 3, embarrassed the administration by concluding that Tehran had halted its weapons program in 2003, which seemed to undermine years of bellicose rhetoric from Bush and other senior officials about Iran’s nuclear ambitions. But in private conversations with Israeli Prime Minister Ehud Olmert last week, the president all but disowned the document, said a senior administration official who accompanied Bush on his six-nation trip to the Mideast. “He told the Israelis that he can’t control what the intelligence community says, but that [the NIE’s] conclusions don’t reflect his own views” about Iran’s nuclear-weapons program, said the official, who would discuss intelligence matters only on the condition of anonymity.
Because, ya know, he’s the Deciderer. Just because he’s been catastrophically wrong in the past shouldn’t be any reason to doubt his judgment now, right?