Why, for thousands of years, did none of God's prophets teach his people about the Trinity?

This is the mystery of the Gospel. The prophets of old wondered and pondered these things but never lived to see God's plan unfold. Now that God has revealed His Messiah, The next great mystery (and we are in the season of its fulfillment) is the rapture of true believers which initiates the 7 last years and Great Tribulation that will happen and effect the whole earth.