Because the differences between Dean and Kerry's positions are all in the eye of the beholder:
Dean:Saddam must be disarmed, but with a multilateral force under the auspices of the United Nations. If the U.N. in the end chooses not to enforce its own resolutions, then the U.S. should give Saddam 30 to 60 days to disarm, and if he doesn't, unilateral action is a regrettable, but unavoidable, choice.
http://www.salon.com/news/feature/2003/02/20/dean/index2.html
Kerry: "If we go it alone without reason, we risk inflaming an entire region and breeding a new generation of terrorists, a new cadre of anti-American zealots - and we will be less secure, not more secure, at the end of the day, even with Saddam Hussein disarmed. Let there be no doubt or confusion as to where I stand: I will support a multilateral effort to disarm Iraq by force, if we have exhausted all other options. But I cannot - and will not - support a unilateral, US war against Iraq unless the threat is imminent and no multilateral effort is possible." http://www.johnkerry.com/news/speeches/spc_2002_1009.html
Dean:"In Iraq, I would be prepared to go ahead without further Security Council backing if it were clear the threat posed to us by Saddam Hussein was imminent, and could neither be contained nor deterred."
http://www.gwu.edu/~action/2004/dean/dean021703sp.html
Kerry:"we should be particularly concerned that we do not go alone or essentially alone if we can avoid it, because the complications and costs of post-war Iraq would be far better managed and shared with United Nation's participation. And, while American security must never be ceded to any institution or to another institution's decision, I say to the President, show respect for the process of international diplomacy because it is not only right, it can make America stronger - and show the world some appropriate patience in building a genuine coalition. Mr. President, do not rush to war."
http://www.johnkerry.com/news/speeches/spc_2003_0123.html
Dean:"never been in doubt about the evil of Saddam Hussein or the necessity of removing his weapons of mass destruction."
http://blog.deanforamerica.com/archives/000395.html