Because a large part of McCain’s appeal is with independent voters, something the war has already eroded. McCain’s best performances in 2000 — indeed, almost all his wins — came in open primaries. Other than Arizona, he lost actual Republicans in every state. That even includes liberal states like Massachusetts, where he won over 60 percent because of huge margins among independents.
Now, McCain ended up losing the nomination for the very same reason — Bush cleaned his clock when the schedule shifted to closed primiaries. So perhaps McCain’s war stance will help him attract more Republican base support. But it is definitely a gamble, because it is based on trying to win over McCain skeptics at the expense of possibly alienating his strongest supporters.