Joseph Cardinal Ratzinger (as he then was) wrote about this article of the creed for the Mariological Congress in March 1995. The essay is reprinted in Mary: the church at the source by him and Hans Urs von Balthasar (Ignatius Press, 2005). He speaks about the centrality of the incarnation to Christian faith:
In manifesting himself, God shows that he is our Lord, not just our idea. His self-manifestation is rightly at the heart of the Creed. The confession of God's history in the heart of man's history is not a departure from the simple profession of faith in God: rather, it is its intrinsic condition. For this reason, the center of all our creeds is a Yes to Jesus Christ: "By the power of the Holy Spirit he was born of the Virgin Mary, and became man." At this article of the Creed we genuflect, because now heaven, the veil surrounding the hidden God, is torn open, and the Mystery comes into immediate contact with us. The distant God becomes our God, Emmanuel - "God with us".
I would imagine that there is also some link with genuflecting to the consecrated Host, for the same reason.
Hugh Latimer expressed a similar opinion in a sermon of 26 December 1552, though in more "English Reformation" terms:
We read a story, (take it as you will, though it be not a true story:) The devil came once into the church whilst the priest was saying mass; and when he was at these words, Et homo factus est, the devil looked about him, and seeing no man kneel down, or bow his knees, he strake one of them in the face, saying, "What! will you not reverence him for this great benefit which he hath done unto you? I tell you, if he had taken upon him our nature, as he hath taken upon him yours, we would more reverence him than ye do." This story is prettily devised; for we should reverence him; we should honour him, and shew ourselves thankful for his inestimable benefits that he hath shewed upon us miserable wretched sinners, in taking upon him our nature.
The footnote says that the story appears in the Vita Christi by Ludolph of Saxony. This was written in 1374, so there is some history to the idea that it is particularly important to show reverence at this point.