The God Almighty has protected the Qur’ān from being tampered with ever since it was revealed. Why did He take the responsibility of protecting the Qur’ān only and not the previous Divine Scriptures?
Various explanations have been offered for this. To my mind, the best among those offered is that the institution of Prophethood existed in the period of all previous scriptures. This meant that divine revelation could still come at the advent of the next Prophet and correct any tampering done by the people in previous divine scriptures. However, once an end was put to the institution of Prophethood, the need arose to eternally protect the message of Allah. So, after the departure of Muhammad (sws), the last of the Prophets, preservation of the Book became essential.