In putting together his dictum, Horace, as a native speaker of Latin, perhaps instinctively chose to put first the word "sapere," and then the word "aude," even if, strictly grammatically speaking, "aude sapere" might have been equally correct.
Did he choose to put the verb in the imperative mood at the end of his dictum because it is a personal mood, like the indicative and the subjunctive, and the verb in the infinitive in initial position because the infinitive is not a personal mood, and we all know that in Latin there is a strong preference for verbs in personal moods to be placed at the end of their sentences?