Why did Allah preserve the Quran and allow His earlier books of divine revelation to be changed or lost?

The answer to that question lies in the following three facts:

The earlier prophets and their books were sent to a particular people in particular periods of history. Once the period ended, a new prophet was sent with a new book to replace the previous book. So, it was not necessary that these books be preserved by Allah. The preservation of the earlier books was left up to the people as a test for them. Thus, when the people went astray, they changed what was written in the books which their prophets brought in order to make allowable the things which were forbidden to them. In that way, all of the earlier books of revelation became either changed or lost.

Prophet Muhammad (peace be upon him) was the last prophet whom Allah sent, and he was not sent to particular people or a particular time. He was sent to all of mankind until the end of the world. Allah said in the Quran,

"I have only sent you (Muhammad) as a giver of glad tidings and a warner to all mankind, but most men do not understand." [Noble Quran 34:28]

Thus, his book of revelation, the Quran, had to be specially preserved from any form of change or loss so that it would be available to all the generations of man until the last day of the world.

The Quran was the main miracle given to Prophet Muhammad (peace be upon him) to prove that he was a true prophet of Allah . So, the Quran had to be saved to prove to the later generations that Muhammad (peace be upon him) was really the last prophet of Allah. All of the false prophets who came after Prophet Muhammad (peace be upon him) brought books which they claimed to be revealed from Allah, but none of them have the miraculous ability to be memorized by thousands, nor have they improved on the message of the Quran. The significance of the Quran's preservation is that Islam has been kept in its original purity because of it. Humanity can always return to the sources of Islam no matter what people may have added or forgotten in time. All of the essential principles of Islam are to be found in the Quran. Consequently, the preservation of the Quran meant the preservation of Islam in its final form. The loss of the Gospel of Jesus means that Christians can never return to the true teachings of Prophet Jesus except by accepting Islam. Similarly, the original Torah was lost when Solomon's Temple in Jerusalem was destroyed by the Babylonians. Thus, the Jews cannot return to the pure teachings of Prophet Moses except by following Islam. It is only in Islam that the pure teachings of the prophets have been preserved without any change. That is why Allah said in the Quran,

"Verily, the only acceptable religion to Allah is Islam."
[Noble Quran 3:19]