I'm sorry to say, but you've got to be the thickest guy on this forum.
While you certainly have little experience racing bikes.
So let me explain it to you reeeeaaaalllll ssssssllllllooooooowwwwwww.
Nibali (the Italian guy, understood?) was forced to give every last bit of energy on the Poggio to try and create a gap, ideally to solo over the top, to then hold on for the victory (or be with only one other rider, slower than him in the sprint),
his only chance at victory. In fact in today's
la Gazzetta dello Sport, Nibali said his attack on the Poggio had been studied at table in the morning with the squadra, but that he knew with those two there was no chance for victory. He also mentioned that the descent from the Poggio was done at a crazy speed, that he suffered and finally tried to get on Fabian's wheel but that Gerrans made it impossible.
Gerrans, your mate, was told by his team to be the stopper on the Poggio, to thus go with anyone who tried to get away, had he the legs, and was successful in his role. (All of which was documented on the RAI interviews.)
Cancellara had to abide by a similar tactic as Nibali, because he too, though not as much as the Italian, was significantly disadvantaged in the sprint.
The split was desperately small, so, quite reasonably, Cancellara took matters into his own hands, being the only one of the trio to be able to motor to the finish with any hope of holding off the others.
Cancellara tried to involve the others in his desparation, though, while Gerrans was able to pull through twice, NIBALI WAS NOT, and said so in the interview immediately after the finish: "Fabain asked for a change,
but then he didn't slow down". When a rider in a situation like that, and after 300 ks, says that the guy who was pulling,
didn't slow down while looking for a change, means that he could simply not pull through. Or if he does pull through, the velocity drops and, with the chasers boring down upon them at 5 seconds, they get caught and surpassed in the sprint.
Capito?
Apart from the these things we have examined so far, the fact that Nibili was unable to come around the others in the sprint, is proof that he wasn't bluffing when he didn't pull through, let alone attack! in the last two ks. He was simply squeezing the last drops of energy out of himself just to stay hitched to Cancellara (and Gerrans), saying it was like riding a time trial, thus at one's limit (
la Gazzetta dello Sport, today).
Capito?
Lastly, as far as the Italian is concerned, besides being without reserves, there wasn't even an incentive to either help or attack, having his teammate Sagan in the chase group so close. This meant had the trio gotten caught, Sagan becomes the man to take the sprint: thus tactically there was no reason for Nibali to assist. But even under the circumstances, had Nibali pulled through, or attacked

as you say, he would have eliminated any hope of winning the sprint for himself, which, as we have seem, wasn't even possible to him even by just sitting on wheels. In fact for a combination of all these reasons, both owing to fatigue and tactics, he decided for himself that under no circumstances would he move to the front. (
la Gazzetta dello Sport, today)
Now either because Gerrans was just being savvy, or he too was not able to maintain the Swiss' impossible rhythm, or, having done his two turns, a combination of both: whatever the case, he hedged his bet well and had just enough gas at the end (but not by much!) to come around the Swiss. Bravo! While even if Cancellara thought he could grind him down, it was a gamble under the dire circumstances he was forced to take. For the Swiss, in any case, had no other choice, and probably knew he was racing for a podium placing (rather than 10th if getting caught from behind), but at the same time in the slightest hope that he may have been able to resist Gerrans in the final sprint. Thus in racing for a podium spot, Cancellara was at once also giving himself the only opportunity for victory. Rather than racing "tactically" correct to position himself best for the sprint by sitting up and forcing the others to move forward, Cancellara choose, quite instinctively, to maintian his relentless pace to prevent the trio from getting caught and losing the podium as well.
Capito?
Moral: Nibali laid it all out on the line on the last climb to play his sole victory card, found two ugly clients he wasn’t able to shake off and, spent afterwards, still held on for third, because of the awesome motor of the Swiss. Fabian, for his part, did everything he possibly could to have escaped the fastest of the sprinters, but found he had to take over the reigns as the only means to not have them come back at the end and got second. The Australian found himself in the ideal, perhaps once in a lifetime situation, for which he was able to take full advantage of the athletic prowess of the others and, of the three having the most speed at the end, took the narrow win. The strongest, however, was Cancellara.