Because it was in our collective self-interest to do so. Had only Vietnamese been dying in the war there, few if any Americans would have taken notice, let alone taken to the streets. I'd like to at least be able to believe we are a moral people, a caring people, a just people, and not ruthless, serial invaders of weaker nations around the world. But the truth is the truth. The only real choice we have in this life is to face the truth and deal with it one way or the other, or to live lives of delusion.
Truth is better.
Delusion can be exceedingly dangerous. The truth of the crime that is being committed against the people of Iraq is that it is not in the self-interest of most Americans stop it. That's why it goes on, no other reason. Were we a moral people, this would not be so. But we are who we are.