Why did punishment not descend upon Quraish after their denial of the miracle of the splitting of the moon, whereas punishment would have descended upon them had they denied the miracle of turning the mountain into gold?

The splitting of the moon was a miracle granted to the Messenger of Allah, peace and blessings be upon him, when the disbelievers of Quraish requested a sign to attest to his truthfulness. The Noble Quran and the Prophetic Sunnah (definition) have recorded this incident. It was not a specific request from them, but rather a demand for proof of his veracity. As for the miracles they explicitly requested, Allah prevented their sending because earlier nations disbelieved in them, and had these miracles come to them and they subsequently disbelieved, punishment would have descended upon them. The Messenger, peace and blessings be upon him, chose to grant them a reprieve.