In Islam it is believed that the Bible was revealed by God but was then corrupted by man.
Does Islam provide an account of why God protected the Qur'an from corruption but not the Bible?
Through empirical evidences we know that there are variations within the Bible. These different variations are sometimes conflicting with other variations.3
Different Churches have different views on which versions are correct. Unlike that of Christianity according to the two major sects that make up the majority of Muslims, Sunni and Shi'a Muslims both agree the Qur'an we have to date is either the exact same text or very similar only differentiating in dialect.
[Background information to help non-Muslim readers understand the Islamic stance on the reliability of the Bible.]
Within hadith we are told why Jews and Christians have edited their text: 1
Narrated Ubaidullah: Ibn `Abbas said, "Why do you ask the people of the scripture about anything while your Book (Qur'an) which has been revealed to Allah's Messenger (ﷺ) is newer and the latest? You read it pure, undistorted and unchanged, and Allah has told you that the people of the scripture (Jews and Christians) changed their scripture and distorted it, and wrote the scripture with their own hands and said, 'It is from Allah,' to sell it for a little gain. Does not the knowledge which has come to you prevent you from asking them about anything? No, by Allah, we have never seen any man from them asking you regarding what has been revealed to you!"
In Surah Al-Hijr [15:9] Allah (swt) has written:
We have, without doubt, sent down the Message; and We will assuredly guard it (from corruption).
So why has Allah (swt) protect the Qur'an but not the Bible?
According to Islamic perspective, this whole world is nothing more but to test us. Allah (swt) put in the Bible a promise (a test). A promise to those who try to tamper His words in the Bible that they would face hell fire. 2
So to sum it up Allah protected the Qur'an but not the Bible, as for the Bible it was a test to see if people were truly "God fearing" and would stand strong to Allah's tests. So he gave the people of the book (Jews and Christians) a test and to put it simply they failed. Allah (swt) protected the Qur'an, indicating that this test was no longer existed for the Muslims.
2: KJV, Rev. 22:18-19
3: The Orthodox Corruption of Scripture [Book by Bart D. Ehrman]