The shift in focus from Abraham to Moses in the Bible reflects a transition in the narrative arc of the Israelites' history.
Abraham is considered the patriarch of the Israelites, as he is portrayed as the father of the nation through his son Isaac, who then became the father of Jacob (Israel). The stories about Abraham primarily revolve around his faithfulness to God, his covenant with God, and the promises made to him regarding his descendants and the land.
The focus shifts to Moses in the book of Exodus, where the Israelites' journey from slavery in Egypt to freedom and covenant with God is recounted. Moses serves as the central figure in this narrative, leading the Israelites out of Egypt, receiving the Ten Commandments and the Law at Mount Sinai, and guiding them through their wilderness wanderings towards the Promised Land.
The shift from Abraham to Moses signifies a transition from the patriarchal era, where God's promises to Abraham laid the foundation for the nation of Israel, to the period of the Exodus and the establishment of the Mosaic covenant, which formed the basis of Israelite religion and society. While Abraham represents the beginning of God's covenant with the Israelites, Moses represents the fulfillment and elaboration of that covenant through the giving of the Law and the establishment of Israel as a nation under God's guidance.