Why was the verse (And my success is not but by Allah) mentioned only once in the Qur’an?

This is an incorrect question and a Muslim should not ask it. Most verses in the Quran are mentioned once, while a few are repeated, such as: "Woe, that Day, to the deniers," and "Then which of the favors of your Lord will you deny?" Allah is not questioned about what He does, but they will be questioned. The servant's duty is obedience and compliance.