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.