Well, consider how many Trinitarian heresies have developed in the age of the Church, even with centuries of solid Jewish monotheism in the background, the little hints at the Trinity that were given gradually in the Old Testament, and Greek philosophy to help make some sense out of difficult subjects. Imagine how much more confusion would have ensued if the Trinity had been explicitly revealed before all these things were fully developed, say during the era when so many Jews were falling into idolatry in the First Temple period
It seems God opted to first work on simple monotheism with the Jews, (again, with a few hints like the vistation of Abraham, perhaps the “Angel of the Lord”, perhaps even the name Elohim, and later writings about Wisdom and the Holy Spirit, so that in retrospect the doctrine of the Trinity wouldn’t seem to have come out of nothing) and philosophy among the Greeks (which also sometimes tended towards more or less monotheistic conclusions), then hit us with great mysteries like Trinity and Incarnation once we had enough background to avoid simplistic misunderstandings of them like Tritheism or thinking of Jesus as a demigod.