Maybe Bush regrets originally making him such a focus of attention, it was always going to be very difficult to get him.
They've changed their tune now though, Bin-Laden is no longer an issue apparently.
Maybe a recognition that he's just a figurehead, like most well-organised, well-motivated terrorist groups, Al-Queda probably has a 'cell' system, each cell able to operate independently of each other and without a top-down command structure.
Also, why highlight him? Maybe a bit embarrassing if it's true that he was allowed to get away from Tora-Bora because the proxy Afghan forces were easily bribed, (you'd have thought that using proxy personnel on critical missions would have ended with the Bay Of Pigs in 1961, if you're going to fight a war, do it, if casualties are such an issue then what are you doing there anyway?)
But Bush could have displayed Bin-Laden's head on a stick, it would not have meant a certain end to terrorist attacks.
Then there is the possibility that he was close to capture, so took his own life, his bodyguards disposing of the body, thus creating a legend, a constant unanswered question for his enemies.
Extensive DNA checks on remains in bombed out Al-Queda caves and bunkers have drawn a blank.
But very likely he and his followers are holed up in a lawless, sympathetic region on the Pakistan/Afghan border, waiting. Waiting for the Western forces to leave, or regrouping to stage guerilla attacks, remember he's a veteran of the insurgency against the Soviets, (with much CIA help at the time).
He also knows the 'instant results' driven impatience of his enemies.