1. According to Ghamidi Sahab, religion starts from the prophet and sometime books are revealed to them but religion always start from a prophet. A book isn’t always needed.
2. So in other case such as Children of Israel, Allah sent prophets to them consistently. There was no need to take special measures to preserve previous books because a prophet would always be present to fix any changes or problems regarding religion.
3. Allah gives revelation and wisdom to people as a gift but if they prove to be disobedient, then as a punishment He sometime takes it away from them as well.
4. Qur’an is the last book, after this, no prophet will come nor a book will be revealed. So it became a need that it must be protected from corruption. At this point, Qur’an functions both as a book and a warner because no warner will come after Prophet Muhammad PBUH.