The main Patristic reason (and mine too) is that our final salvation requires a Triune God. The Unitarian God cannot ultimately save. Once this is perceived and experienced, its clarity grows. I’ll try to express my own understanding of this in three key claims:
(1) God cannot give us in salvation that which he doesn’t possess.
(2) What is not adopted is not healed.
(3) The salvation of humanity is finally achieved through participation in God’s being and life (2Pet 1.4).
Working backwards from (3), human salvation isn’t the achieving of a legal status granted by divine fiat. It is in the end nothing less than the perfection of our natures, our actually becoming, in relationship to God, all that he intended us to be. God saves us not by a wave of the divine wand and simply declaring it to be so. We are finally saved/perfected in union with God, in relationship to him whose own existence and life achieve and ground the abiding perfection of our natures. One friend recently insisted that God is free to forgive us apart from incarnation and the Cross. Quite true, though entirely beside the point. Final salvation is so much more than forgiveness.
On to (2). By what means is created human being to participate in the life which saves, that is, in the life of uncreated divine being, and so find its final fulfillment? The two have to be united. God must adopt or take up human being into his own life. So incarnation becomes essential to the redemption and healing of humanity (contrary to those who suppose that our final and fullest perfection in God is conceivable apart from incarnation). If our salvation is participation in divine being, and if our participation in divine being requires incarnation, then incarnation is the ground and means of our salvation. Incarnation saves, and it saves because our created nature which requires relationship to and union with the uncreated God is taken up by him personally and irrevocably.
Which brings us to (1). Is God essentially, within himself, that life and love sufficient to save and fulfill us? If our final salvation requires participation in the life of God, then the question is — Is God’s life sufficient to save? Is God’s way of existing/living what we require to exist/live in the fullest sense possible? And this is where Unitarianism is exposed as non-Christian and void of saving efficacy, because a Unitarian God cannot be (in and of Godself, essentially) a loving and personal being. How can a solitary, unrelated Unitarian God whose existence and essential experience by definition are void of the personal address and response definitive of love and personal being (“I/Thou”) be that which bestows fully relational and loving existence upon us? A Unitarian God stands alongside created individuals as needing that which bestows fully personalized, loving existence. The Fathers saw this, which is why their understanding of salvation as fully realized loving/personal existence, the kind of existence and life which are essentially God’s, available to us through Christ, eventually and naturally (and rightly) led to a trinitarian understanding of God.
Think of all that open theists have argued so strenuously about God being (essentially) love, and love as essentially relational in nature. None of that can be true of God if Unitarianism is true, for there is simply no conceptual mechanism within Unitarianism to ground God’s essential existence as perfect love and God as an irreducibly personal being independent of creation.
If God—the One true God—isn’t a God who is love, who is essentially a fully realized personal and loving being apart from all created being—then God is not in himself that which saves us and if God is not that which saves us, we are lost.