The second reason is Republicans are routinely more eager to go to war than Democrats are. There may be times when going to war is the right thing to do, but Iraq was not one of them.
The third reason is something of a combination of the first two: Republicans encourage a structure of government and economy that induces a national addiction to war. If there is no war, then Dick Cheney does not get rich.
The fourth reason is that the Democrats are more supportive of human rights than Republicans.