Commenter vernezze has an intriguing if somewhat conspiratorial theory for why McCain won't mention Jeremiah Wright (one that plays to one of the Stump's pet fascinations, no less):

Call me crazy (but at least wait until you read the post) but I believe that Powell has made a deal with McCain. I will not endorse Obama in this election if you do not bring up Wright. How else to explain the dog that does not bark in the face of very favorable things he has said about Obama in the past and the adumbrations from reliable sources that a Powell endorsement for Obama was on the way? ... I admit that the only evidence I can claim I get from Sherlock Holmes: when all else has been eliminated, what remains, no matter how unlikely, must be the case. Of course, events could disprove all this should either McCain start the Wright attack or Powell come out in favor of Obama. But until that point, I am sticking to my guns.

I've heard crazier things.

--Noam Scheiber