According to Osho, Tantra was a powerful, scientific path to transform sexual energy into enlightenment, but sex-repressive society violently persecuted tantrikas—massacring couples, burning scriptures, driving adepts into forests. To preserve the living stream of methods, masters chose "whispered transmission": pass it quietly, person-to-person, not publicly. In India, where many sages existed, martyrdom stirred little outrage, so secrecy became the only practical way to keep Tantra alive.
They whispered because open teaching got them killed, so they shared the wisdom quietly to keep it safe and alive.