Why did Allah allow other books beside Quran to be changed by man like the Torah, gospel and bible? It was his own sacred word too when they were revealed and they were still his words so why did he allow that to happen? Why they became secondary when at one point they were the most important books for mankind?
The answer is the same as why God allowed Adam to be tricked by Satan into leaving Paradise.
This universe is a story that God writes. He gives humans things and sees what they do with it, and based on that, He responds and gives them new things, in this way writing a story that is truly authentic and made up of the free choices of millions of people.
This instant, God could solve all of the world’s problems and turn it into Paradise. He does not, because the point of this world is the story, the process, not the destination. This process continually produces good, sincere and proven friends of God who die and spend eternity in closeness to Him. If these people had been brought up in Paradise instead, never suffering, they would have never had any chance to prove their sincerity, their patience, their courage, since all of these require hardship, temptation and the existence of evils.
God is the Master and Teacher of humanity. Humans used to be ignorant, Stone Age people. God gave the Hebrews the Torah, in this way teaching them, helping humanity grow, giving them new ideas and new ways of social organization.
He knew that they would eventually corrupt it, and He allowed it to happen, because this makes a good story. It creates the opportunity for sending new prophets and new revelations, in this way continuing the growth process of humanity, raising them the way you raise a child. A good teacher does not control everything the student can do, and does not prevent all possibilities for making mistakes.
God continued to send new prophets to the Jews for a thousand years, yet during all of this time they continued to persecute their prophets, reject them and at times stone them to death.
God then sent Jesus, 
, a Jewish prophet meant to be a witness against the Jews, and meant to be the starter of a world-wide movement that would forever change history.
To continue this story, God sent Prophet Muhammad, , as an answer to a prayer that Prophet Ibrahim made 2500 years before Prophet Muhammad was born:
127. As Abraham raises the foundations of the House, together with Ishmael, “Our Lord, accept it from us, You are the Hearer, the Knower.
128. Our Lord, and make us submissive to You, and from our descendants a community submissive to You. And show us our rites, and accept our repentance. You are the Acceptor of Repentance, the Merciful.
129. Our Lord, and raise up among them a messenger, of themselves, who will recite to them Your revelations, and teach them the Book and wisdom, and purify them. You are the Almighty, the Wise.” (The Quran, verses 2:127-129)
The sending of Prophet Muhammad was an insult and punishment to the Jews by God, because while he is descended from Ibrahim (as the Jews were promised their final prophet would be), he was not a Jew. All Jews are descended from Ibrahim, so they thought their final prophet would be too. But God instead raised the final prophet from Ismail’s descendants (the Arabs of Hijaz), who, while being descended from Ibrahim as promised, was not Jewish.
87. We gave Moses the Scripture, and sent a succession of messengers after him. And We gave Jesus son of Mary the clear proofs, and We supported him with the Holy Spirit. Is it that whenever a messenger comes to you with anything your souls do not desire, you grew arrogant, calling some impostors, and killing others?
90. Miserable is what they sold their souls for—rejecting what God has revealed, out of resentment that God would send down His grace upon whomever He chooses from among His servants. Thus they incurred wrath upon wrath. And there is a demeaning punishment for the disbelievers.
93. And We made a covenant with you, and raised the Mount above you: “Take what We have given you firmly, and listen.” They said, “We hear and disobey.” And their hearts became filled with the love of the calf because of their disbelief. Say, “Wretched is what your faith commands you to do, if you are believers.”
94. Say, “If the Final Home with God is yours alone, to the exclusion of all other people, then wish for death if you are sincere.”
95. But they will never wish for it, because of what their hands have forwarded. God is aware of the evildoers. (The Quran, verses 2:87, 2:90, 2:93-95)
The reason why God allowed His scriptures to be corrupted is that it was useful in the raising of mankind, that it made a good lesson for those who came after, and that it was a test to those who did it. It could have served many other purposes that we will never know about. God decided to protect His final message from corruption because the corruption of scripture had already served its purpose, and God had a new purpose in the Quran.
We are like ants who are being managed from above, being turned this way and that, as God wishes, for His own purposes. But we are free-willed, we can disobey Him and rebel against Him. Regardless of whether we are good or evil, at the end of the day He continues to be All-Powerful above us all, so that we can never escape Him or corrupt the story, the plot line, He has devised for humanity. We can choose what role we play in His story, whether as good people or evil people, but the story is always His story.
God does not change the condition of a people until they change what is within themselves. And if God wills any hardship for a people, there is no turning it back; and apart from Him they have no protector. (From verse 13:11 of the Quran)
As the above verse and many other verses of the Quran teaches, it is God Who manages world history. If we are good and kind, God will respond by changing our conditions for the better, and if we are not, He can send us hardships. We have zero power to ensure our own good, it is all from God. All that we can do is be good internally, and encourage others to be good, and use common sense in our dealing with the world, and the rest is up to God. No matter how hard we try to get something, if God does not want us to have it, we will never get it, which is why it is so important to not be attached to this world, but to God, knowing that all things come from Him, and that nothing can ensure our good other than His decree.
The complexity of this world, the corrupted scriptures, the various religious movements, the knowledge we have and the knowledge we lack, all of these provide an ideal testing ground for humans, separating the truly good and faithful from the average believers, and these from the lost and the evildoers.
God let all previous scripture get corrupted because it makes a good story and therefore new prophets come? But for the past 1400 years he hasn’t let it get corrupted? According to your theory The Quran should be corrupted by now because it makes for a good story. Suddenly God grew tired of having a good story so he chose Muhammad to be the last prophet? I’m sorry but what you are trying to say, makes absolutely no sense what so ever to me, so can you please explain a little better why God got bored and ended the corruption, with Muhammads revelations are unable to be corrupted?
The Quran says no one can alter God’s word, then how was God’s previous revelations able to be altered? When saying that the Bible is corrupted, you are in fact contradicting what the Quran actually says.
How do you know the Quran you read today is the same as it was in Muhammads time? The Quran was not written by Muhammad, but it was composed of his sayings and actions after Muhammad had already died. This is the exact same way the bible came to be, after Jesus died and his followers wrote the new testament scriptures about Jesus’s sayings and actions. So if the Bible is subject to corruption because it was compiled by men after Jesus died, the very same can be said about the Quran because it too just like the Bible was composed by men after Muhammad was dead.
God gave humans the freedom to corrupt the previous scriptures. He could have prevented this, but He didn’t, since they were not meant as final revelations. The Quran, however, since it was meant as a final revelation, was protected by God. It doesn’t make a good story for it to be corrupted because then humanity would be left without any guide (since God is not planning to send any more prophets), so the part of the story in which we live requires that the Quran should not be corrupted.
If the previous books were not corrupted then God would not have sent prophets after prophets especially for the Jews. But now God sent his final prophet with his final message, it’s logic that this time God will protect his final message from alterations, because it’s for everybody.
Note that Jesus was the final prophet for the Jews and Muhammad is the final prophet for mankind, because next is the day of judgement.