Lots of reasons.
*I don't want to see Mr Bush hand the Left the issue they've been waiting to coalesce around;
*I don't want to see America throw away its traditional position of being on the side of what's right, just, and good, in this cynical fashion;
*I don't want the US to become an international pariah, and;
*I don't think Iraq is worth one dead American, Brit, or Aussie boy.