Because he's falling, and the campaign hasn't managed to find a successful new line against McCain in two months. Look at the InTrade graph:
Obama got very little bounce from his convention despite an astouding television and live viewership for his acceptance speech. The McCain contract, on the other hand, got a 1.3 bounce this morning before he even spoke.
When I say that the Obama campaign is in trouble, I don't mean by any means that a McCain victory just became inevitable, or even probable. But Obama is no longer going to have the walk everyone was expecting three months ago. They need to find a more successful means of attacking McCain than they have so far come up with, because the overall trend of their polls is downward, and I'm told that many internal polls, which project likely voters, already show McCain with at least a 50/50 chance of winning. Obama's got a fight on his hands.