I think QB mentality is huge. With great coverage, I think the QB realizes that it is up to him to make the best throws. Perfect offense beats perfect defense every time. When faced with tough coverages, QBs know they have to rise above.
The mentality is completely different when you get pressure on the QB. How many times have you seen a QB completely mentally check out of a game and start screaming at their offensive line when consistent pressure occurs? When you get pressure on the QB, I think you make a QB feel powerless (it's not my fault, it's my protection's fault) and that's going to lead to more mistakes.