I will offer a different answer to this question:
We have revealed the Torah, wherein is guidance and light. The submissive prophets ruled the Jews according to it, so did the rabbis and the scholars, as they were required to protect God’s Book, and were witnesses to it. So do not fear people, but fear Me. And do not sell My revelations for a cheap price. Those who do not rule according to what God revealed are the unbelievers.
Bani Israel had the task to provide God’s Book, and the submissive prophets ruled the Jews according to it, so did the rabbis and the scholars and were witnesses to it.
They did their best to preserve the Word of God. They preserved the Decalogue, the direct written Word in original for 800 years, and in copy until now. They preserved the traditions of the ancient prophets and the hadith of Moses (ﷺ) and the Word revealed to the later prophets until now in the original language, for over 3000 years. Neither Muslim nor Christians did that.
The Christians were a bit less performing; the Gospel that is only related in hadith has only been preserved in translation because the knowledge of the original language was soon lost among them. The fact that the message gone through Jesus (ﷺ) was related in hadith has a good reason: This message is not only contained in the words from his mouth but equally through his deeds and signs. And finally, the Christians preserved more of the revelationcome through Jesus (ﷺ) than we did.
The problem of all of us is that we are not able to discern clearly between truth and imitation, we only try to do so. The same happens in Islam with our hadith: the scholars had to decide with their limited capabilities whether a hadith should be considered true.
Now, why was the Quran preserved?
Because the people were fully aware of the task to preserve it in its original wording already during the life time of Muhammad (ﷺ). This is how it has been protected. God has not protected the Quran beyond our possibility but by means of our possibilities.
The Quran is the last scripture revelation. This is a good reason why it has been protected more diligently than other revelation before.
Philosophical background of this Answer:
I part from the working hypothesis that anything that can be explained from our own experience is more probable than an explanation that cannot be experienced in our lives.
What is a working hypothesis?
A working hypothesis is an unverified assumption. There is no claim for final truth behind it. Indeed, I am aware that the aforementioned hypothesis cannot be upheld as a dogma (teaching claiming to be true) on the backgroud of the Quran; it is just a criterion for the most probable explanation.
In this case: In contrast to the ability to discern good from evil, which is somehow engraved in the hearts of any human (although there are certain differences according to our religious background), we, in our state and time, are not able to decide whether something is really true (modern: journalistically correct, or in older terms: reliable tradition, to be understood literally or metaphorically) or false (modern: fake news, or in older terms: fabricated hadith).
Now, parting from this experience: How can the grade of the preservation of the Word of God in the Torah, the Prophetic scriptures, the writings of the minor Jewish Nabi, the Gospel accounts, the Quran and the Hadith be explained?
- All those who contributed to the preservation were essentially believers at least in their own belief (a Jew who lived before Jesus (ﷺ) could not be a disbeliever according to the message that Jesus (ﷺ) was the Messiah, and a Jew or Christian before Muhammad (ﷺ) could not be a disbeliever rejecting him as a prophet; and even those who did not believe in Jesus or teh two lates prophets acted according to their belief). On the basis of theirs beliefs, it has always be important to them to preserve the Word of God as good as they were able to do so.
- With our hadith we can see exactly our disability to discern true from false: The ancient (and partly even the contemporary) scholars have to decide on rational criteria such as the chain of transmission, the compatibility with the Quran, the reliability of any person of the chain and indications whether the subject was really known in the times of Muhammad (ﷺ). Nobody can decide with certainty, reading a hadith: this is true, and this is not.
- It is right to assume that the people before faced the same difficulties.
- In spite of the fact that the Arabic writing in the times of Muhammad (ﷺ) was extremely deficient (not even consonnant punctuation!) so that a written text was little worth safe one knows what had be written, just serving as a reminder, the companions were fully aware that the Quran will be a Book, the last Book of Revelation. The Quran is even called a Book in itself, although in the time of revelation, it was not a book at all. This awareness of the importance of the revelation can - without any supernatural force - explain the effort and the success of the companions of Muhammad (ﷺ) to remember the words in mind and by means of written notes, so that it could finally become the book we hold in our hands.