I wonder why God did not reveal his name as Jehovah Yahweh to them

well, he did. for instance, when he introduces himself to abram:

וַיֹּאמֶר אֵלָיו אֲנִי יְהוָה אֲשֶׁר הֹוצֵאתִיךָ מֵאוּר כַּשְׂדִּים
and he said to him, "i am yahweh that brought you from ur kasdim (chaldees)..." (gen 15:7)

there's several other places, too. in fact, he introduces himself to moses just three chapters earlier as "yahweh".

the problem is that we're looking at a composite text: someone has taken two sources and mashed them together. one version of the story has it that everyone has always known who yahweh was, and the other has it that yahweh revealed himself as yahweh first to moses.

the historical reason for this is the conflation that went into making yahweh in the early israelite cults. the god he identifies himself as -- el shaday -- is pretty clearly the analog for the canaanite high god, el. yahweh seems to have initially been a lesser god in the early pantheon, and took over as the primary monolatrist god (a "baal" if you'd like) sometime around the 10th century BCE. the question then becomes "is yahweh the same god as el?" and "when did we know this?" one history is revisionist and asserts that it was always this way, and another terms it as revelation.

and who translated the verse and why they added to it?

there are traditions about god having many names, and the KJV is probably reflecting that, or maybe translating some emendation ("footnote") or scribal addition somewhere. hard to say.

strictly speaking, at one point "el" was the name of a god, in canaan, but by the time the bible is being written, it is simply a generic word, as is "elohim".