Why did Socrates triumph?

In Twilight of the idols, N talks about how Socrates brought down the dichotomy of Dionysia-Apollonian and brought up the dichotomy of reason-unreason, thus paving the way for nihilism. N holds Socrates responsible for this.

However, in the same book, under the chapter four great error, Nietzsche talks about mixing cause and consequence: "The Church and morality say: “A race, a people perish through vice and luxury.” My reinstated reason says: when a people are going to the dogs, when they are degenerating physiologically, vice and luxury (that is to say, the need of ever stronger and more frequent stimuli such as all exhausted natures are acquainted with) are bound to result. Such and such a young man grows pale and withered prematurely. His friends say this or that illness is the cause of it I say: the fact that he became ill, the fact that he did not resist illness, was in itself already the outcome of impoverished life, of hereditary exhaustion. The newspaper reader says: such and such a party by committing such an error will meet its death. My superior politics say: a party that can make such mistakes, is in its last agony—it no longer possesses any certainty of instinct. Every mistake is in every sense the sequel to degeneration of the instincts, to disintegration of the will. This is almost the definition of evil, Everything valuable is instinct—and consequently easy, necessary, free. Exertion is an objection, the god is characteristically different from the hero (in my language: light feet are the first attribute of divinity)."

So, according to Nietzsche, Socrates must be just a symptom. If the Ancient greeks' instincts hadn't already degenerated, he would never be able to penetrate their minds. What happened?