1. Why one god?
Because, if there is a single source of all creation (or, all order) then there is unity of reality. And unity of reality is necessary for real objective understanding.
In fact there does not need to be one god, but one source: God*.
2. Why God the Father?
Because this explains why God is concerned with us, as individuals - out of the whole of reality. We are God's children; and like mortal children it is the hope and destiny we grow up to become of the same 'kind' as our Heavenly Father.
3. Why a God of Love?
Because otherwise, even if we were His children, we could not assume that God's intentions were benign and there would be no reason to go along with God's intentions. It is only a Loving Father whose plans we would wish to assent to.
4. Why mortal earthly life?
This life is mixed, full of change and decay and also love and hope. Mortal life seems insignificant in an eternal perspective, yet feels overwhelmingly significant in the here and now. There must be some benefit from mortal life, or else a loving creator Father would not have made the situation; yet death puts an end to all benefit we might gain from living, and our Father also made death...
Therefore, we infer mortal life is necessary and death is necessary; both necessary, that is, for God's hopes and plans.
5. Why Christ?
Because death is necessary - yet death is the end. We need to die; and Christ is our Saviour from death.
6. Why Jesus?
Because Jesus is our brother; therefore he both shows us the way, and by preceding us makes that way so we can follow.
*For Mormons the unity of a single God is not that of a single being or entity; but a single, inseparably unified but eternally dyadic creative-marriage of Heavenly Father and Mother. So mono-theism (one god-personage) is not entailed - but one divine-source is.