The Tetragrammaton, which is the Greek name for it and not the name itself, is forbidden for Jews to utter because it is the highest and only true name of God. This started a century or two before the time of Jesus, when the Priests at the Jewish Temple in Jerusalem made a power play. They forbade the average Jew from speaking the name, and declared that only they could speak it, and only during high ritual ceremonies at the Temple. Before that time, any Jew was free to utter the name any time he or she wished. The priests saw a chance to gather in more power and authority for themselves, and they took it. Sort of similar to Jews telling whites that we cannot speak the word "nigger" -- that only niggers can say the word. A power grab. And then, the Jews murdered Jesus, the only-begotten son of God, by manipulating the Romans into crucifying him, and God in his wrath used the Roman army as his instrument of divine judgment to destroy the Temple. The Romans leveled it down to the foundations, and then destroyed the houses of rebellious Jews and drove them out of Jerusalem. All this was the punishment of God on Jews for the sin of manipulating the Romans into executing Jesus. After that, there were no more priests, there was no more Temple, and no Jew could speak the Tetragrammaton. And that's how it has been for two thousand years, and how it remains to this day.