“Moses received the Torah at Sinai and transmitted it to Joshua, Joshua to the elders, and the elders to the prophets, and the prophets to the Men of the Great Assembly. They said three things: Be patient in justice, raise many disciples and make a fence around the Torah.” Avot 1:1
I'm aware of the fact that the Mishnaic tractates had originally no order, but they were eventually structured into SHAS as we know it today. I'd expect this Mishnah to be the opening sentence of the whole work as it explains the origin and the authority of the whole Mishnah. Yet it is buried amidst Sanhedrin.
In addition, I don't see a prohibition of duplicating this Mishna, so it could appear both at the beginning of Berachot AND Avot to assert that both Halachah and ethics were given at Sinai.
Why isn't this Mishna used as the introduction to the whole Oral Torah?