What was the reason why the Jews specifically used the substitute Adonay/Kyrios for the Tetragrammaton? God has one name. ‘’I am Yhwh and that is my name’’ (אֲנִ֥י יְהוָ֖ה ה֣וּא שְׁמִ֑י Isaiah 42:8).

In the Hebrew Pentateuch the name of God was the Tetragrammaton (the four letter) ‘Yhwh’.

The Jews respected the Hebrew name of God so much to the point of actually using a substitute word for the divine name which was ‘Adonay’ (my Lord) whenever they spoke of it or read of it in the Tanakh. The exilic Jews followed this tradition of using a substitute for the divine name and they translated the name of God as Kyrios in Greek as found in the Septuagint. Kyrios simply means ‘master, owner, lord’.

What was the reason why the Jews specifically used the substitute Adonay/Kyrios for the Tetragrammaton?
Here is an answer from a Jewish perspective.

The Bible calls G-d Y-h-v-h, which is called “the Tetragrammaton,” which means a four-letter word. While the Tetragrammaton appears on Torah scrolls, even today, Jews say it as Adonai whenever they read the Torah (more on that later).

Many think that this is the name of G-d in the Hebrew Bible, but this is not true. It is a description of how G-d functions in the universe. Thus, when Moses asks G-d in 3:13, the people “will say to me, ‘What is His name?’ what should I say to them?” G-d replies, “‘I am that I am’." Virtually all Jewish commentators and rabbis understand verse 14 to be saying that this is describing how G-d functions or acts. It is not a name.

Since ancient times, Jews have avoided using the Tetragrammaton out of respect around 250 BCE. When they translated the Bible into Greek in Egypt, they substituted the Tetragrammaton for the Greek word Kurios, which means “Lord.” Around 250 the Jews began to substitute Kurios for the Hebrew version of Kurios, Adonai, which also means “Lord.” Eventually, Jews began to treat Adonai as the Tetragrammaton and reverted to the new circumlocution Hashem, meaning The Name. Out of respect, Jews no longer use Adonai, substituting Hashem in its place. Yet others went as far as to write the word "God" as G-d.

Similarly, the word “Elohim” is not a name for G-d but a function. In English it is G-d. The word el is the singular of Elohim, which means “powerful.” Idols, too, is called el, because the ancients felt that these idols were also powerful. G-d is called Elohim (plural) in the Bible because G-d is more powerful than anything else.