Why is the word "LORD" written in uppercase letters?

The title Lord in capital letters signifies that this is the proper name of God, Yahweh, by which God identified himself to the Israelites in Exodus 3:14–15. By this name, God reveals himself as the God of the covenant, that intimate relationship of love that binds God to his people, and in which God demands loving obedience in return. In the four chapters of this prophecy, this name of God appears forty-one times, highlighting the importance of the covenant relationship that forms the foundation of this prophecy.