When he attacked most riders were preparing for the final kms. Some even back at their team cars. No one expected his attack. I am not arguing that he wasn´t the strongest. He clearly was. Even when the group split he continued to increase the gap to Fuglsang, Yates and Majka. I am just saying that all of them were caught sleeping by his initial attack. Flat road. Over 50kms to go.
For whatever reason they still underestimated him. Based on his strength he should be the marked rider (think peak Cancellara) but for whatever reason he isn´t. Once he has a lead there is no way to bring him back.
It's not as clear cut as that though. He attacked at the moment that a previous break had just been brought back, the road was going slightly uphill, everybody needed to catch his breath. Everybody but him. Then he attacks 51k from the finish, with a big climb coming up. He knew when to attack, because the others needed to pause a moment, and there was also a supply zone. This is what gave him the initial gap of 50 meters. The fact that it was still a long way to go, and the climb was coming up, makes the riders hesitate. Who wants to burn his matches? Not a lot of domestiques around, and the responsibility lies with the race leader, Carapaz, first and foremost, who still had a domestique (Dunbar). This hesitation gave him an extra 50 or so meters. But you should look at the race again, because before he starts to gradually widen the gap, there is actually fierce response, the first minute, at a moment he only has 10 seconds. Until they understand that they won't simply bridge without burning a few matches, and nobody is willing to do that, so they look at Carapaz and Dunbar. At that moment, the race was over.