Because it's a masterstroke. With Carter's abject record of humiliating failure in dealing with middle-eastern rogue states, there's only upside for Bush in having Carter on the other side. This op-ed will produce no new opposition to the war, as everyone capable of being convinced by Jimmy Carter on this issue is already against the war anyway. For everyone else, it's a reminder of what the politics of appeasement look like, and where they lead.
UPDATE: And don't miss Eliot Cohen's flaying of the "chickenhawk" slur:
There is no evidence that generals as a class make wiser national security policymakers than civilians. George C. Marshall, our greatest soldier statesman after George Washington, opposed shipping arms to Britain in 1940. His boss, Franklin D. Roosevelt, with nary a day in uniform, thought otherwise. Whose judgment looks better? A few soldiers become great diplomats or great politicians; others are abject failures. Most avoid the field altogether. Military careers spent in hierarchical, rule-bound, tightly controlled organizations are not necessarily the best preparation for accurately judging the fluid world of politics at home and abroad.
There's more, and it's good. He even mentions Starship Troopers.