There is no other transitionary event in human history that warrants three full months of focused attention and persistent acknowledgment than the incarnation of the Son of God.
In Advent we wait, in Christmas we rejoice over the coming of Christ in the fulfillment of the promises, and in Epiphany we celebrate the surprise, the manifestation of Christ to the Gentiles.
The Messenger is coming—must come—because the LORD God has promised, and He is unchanging and always faithful no matter how unfaithful His people may be.
In Genesis 1-2, the Lord reveals—or, at a bare minimum, starts dropping some big hints—that he will be quite comfortable becoming a human being himself someday.