Why God Gave Moses the Oral Torah by Mouth Alone

(Exodus 34:27:) "And the LORD said to Moses: Write for yourself these words." This is what Scripture says (Hosea 8:12): "I wrote for him the greater part of My Torah." When the Holy One, blessed be He, came to give the Torah, He spoke it to Moses in order: the Scripture, and the Mishnah, and the Aggadah, and the Talmud, as it is said (Exodus 20:1): "And God spoke all these words." Even what a seasoned disciple would in the future ask of his teacher, the Holy One, blessed be He, told to Moses at that hour, as it is said: "And God spoke all these words." After Moses had learned it, the Holy One, blessed be He, said to him: Go and teach it to My children. Moses said to Him: Master of the Universe, write it down for Your children. He said to him: I would wish to give it to them in writing, but it is revealed before Me that the nations of the world are destined to rule over them and to take it from them, and My children would then be like the nations of the world. Rather, give them the Scripture in writing, and the Mishnah and the Aggadah and the Talmud by mouth. (Exodus 34:27:) "And the LORD said to Moses: Write" — this is Scripture. "For by mouth" (Exodus 34:27) — the Mishnah and the Talmud, which set Israel apart from the nations of the world.