Reasons

A good observation from Ezra Klein on Barack Obama's foreign policy address. Obama says of Iraq:

In 2002, I stated my opposition to the war in Iraq, not only because it was an unnecessary diversion from the struggle against the terrorists who attacked us on September 11th, but also because it was based on a fundamental misunderstanding of the threats that 9/11 brought to light. I believed then, and believe now, that it was based on old ideologies and outdated strategies – a determination to fight a 21st century struggle with a 20th century mindset.

As Ezra remarks, "What's telling, however, is what's absent. Obama doesn't say he opposed the war because of a nagging skepticism towards Hussein's WMD capabilities, nor because this administration wasn't competent enough to pull such a conflict off. Rather, he opposed it because it was the wrong war, focused on the wrong threats, and stemming from the wrong ideology." Contrast this with, say, John Edwards in his famous "I was wrong" op-ed:

Almost three years ago we went into Iraq to remove what we were told -- and what many of us believed and argued -- was a threat to America. But in fact we now know that Iraq did not have weapons of mass destruction when our forces invaded Iraq in 2003. The intelligence was deeply flawed and, in some cases, manipulated to fit a political agenda.

Obama didn't go on to draw any broader programmatic distinctions between himself and other Democrats, preferring to stay within the formal "positive vision" framework, but it'll be interesting to seee as we get some Democratic debates whether any larger doctrinal differences emerge, or if this is just a question of emphasizing different aspects of the same negative view of the Iraq War.