they cause you to sin against Me: God commanded Israel to destroy the Canaanites. There could be no peace agreement between a holy God and sin.
God had chosen Israel to carry his message (revelation, meaning what God makes known about himself) to the world. This message would one day lead to the coming of Jesus Christ in human form (the incarnation).
For this plan to continue, Israel had to remain faithful to God. If they returned to pagan worship (the worship of false gods), this plan would be broken.