The Jews were in such awe of God's name, that they developed the convention of NEVER pronouncing it out loud. Although the written Hebrew text remained Yahweh, the reader substituted the word "Adonai" and read that aloud to their synagogue congregation. The word "adonai" simply means "lord," a word reserved for addressing aristocrats. 

When the Jews translated the OT into Greek, they substituted "kyrios" (Greek for lord) for the Hebrew Yahweh. Early Christians adopted the Greek NT, and thus inherited this translations "error." When the Greek was translated into Latin, we got "dominus" (Latin for lord), and when the Latin was translated into English we got "Lord."

However, modern translations are direct from Hebrew to English, so translators have to decide between truthfully translation Yahweh as Yahweh, or adopting the Jewish convention.

I suspect, but have not checked, that the substitution of "God" for "Lord" is an attempt at a middle ground. Lord is wrong; that is not God's name.