1- Allah did not make such a promise with the Taurat and Injeel
2- The Qur’an is a continuation of Taurat and Injeel, and by protecting the Qur’an Allah has protected the message of the Taurat and Injeel.
3- The Qur’an is Allah’s final scripture. There won’t be another scripture to come and correct if it gets changed (unlike the Taura and Injeel—as another scripture came after them to correct what had been changed). Since it’s the final one Allah wanted to protect it so the truth remains.