The question is based on a false presupposition which is that people for 600 years thought that Jesus was crucified.
There are many sects of early Christianity that didn’t believe Jesus was crucified
Not Only the Quran but also Many people in History have claimed that Jesus was neither killed nor crucified.
Non-Muslims Historians and some early Christians testify that Jesus was not killed nor crucified.
In Christianity, Docetism is the doctrine that the phenomenon of Jesus, his historical and bodily existence, and above all the human form of Jesus, was mere semblance without any true reality. Broadly it is taken as the belief that Jesus only seemed to be human, and that his human form was an illusion.
One of the most accurate translations for Philadelphians 8:2 clearly tells us that this group’s contention was centred around Ignatius documents being corrupted and rejection of the crucifixion narrative peddled by Ignatius since it went against their own original document(s) they had in their possession  :
“18 For I Trust in the grace of Jesus Christ that he will free you from every bond. 19 Nevertheless I exhort you that you do nothing out of strife, but according to the instruction of Christ.
20 BECAUSE I HAVE HEARD OF SOME WHO SAY, UNLESS I FIND IT WRITTEN IN THE ORIGINALS, I WILL NOT BELIEVE IT TO BE WRITTEN IN THE GOSPEL. And when I said, It is written, they answered what lay before them in their CORRUPTED COPIES. 21 But TO ME JESUS CHRIST is instead of all the uncorrupted monuments in the world, together with those undefiled monuments, HIS CROSS, AND DEATH.
And resurrection, and the faith, which is by him, by which I desire, through your prayers, to be justified.” (The genuine epistles of the Apostolic fathers, St. Clement, St. Polycarp, St. Ignatius, St. Barnabas; the Shepherd of Hermas, and the Martyrdoms of St. Ignatius And St Polycarp, [Hartdford: Published by Parsons and Hills, 1834] Translated by William Wake, Lord Archbishop of Canterbury. page 149)
Christians you might deny this above-Mentioned evidence, well that is your problem, you have been denying lots of evidence.
Christian teachings, some Islamic traditions teach that Jesus ascended to Heaven without being put on the cross, but that God transformed another person to appear exactly like him. This thought is supported in misreading an account by Irenaeus, the 2nd-century Alexandrian Gnostic Basilides when refuting what he believes to be a heresy denying the death.
According to the Second Treatise of the Great Seth, Yaldabaoth (the Creator of the material universe) and his Archons tried to kill Jesus by crucifixion, but only killed their own man (that is the body). While Jesus ascended from his body, Yaldabaoth and his followers thought Jesus to be dead. In Apocalypse of Peter, Peter talks with the savior whom the “priests and people” believed to have killed.
Manichaeism, which was influenced by Gnostic ideas, adhered to the idea that not Jesus, but somebody else was crucified instead. Jesus’ suffering on the cross is depicted as the state of light particles (spirit) within matter instead.
According to Bogomilism, the crucifixion was an attempt by Lucifer to destroy Jesus, while the earthly Jesus was regarded as a prophet, Jesus himself was an immaterial being that cannot be killed. Accordingly, Lucifer failed and Jesus’ sufferings on the cross were only an illusion.
Even according to some Christian sects in Japan, Jesus Christ did not die on the cross at Golgotha
According to the Glorious holy Quran, Jesus was not killed nor died or crucified by his enemies. The enemies of Jesus created the deception of crucifixion as a propaganda tool to subvert his good mission to their advantage. If Jesus is crucified then the devil has won, and so the devil also rules the world.
There are always and have always been two aspects of any war: a physical war and a psychological war. A physical war usually does not last very long, but the psychological war has no limits of time and space.
The story of the war of Jesus represents a psychological battlefield where the forces of ‘Goodness’ and ‘Corruption’ fight for the supremacy of their principal arguments. Those who want the crucified Jesus supports the Corruption side to justify their own corrupt actions under the pretense of survival.
The main excuse for any form of Corruption is always Survival, whilst the Goodness cares only about Compassion. If Jesus dies then the Corruption survives, and if he lives then Compassion wins. The narrative of Quran stands for the values of Compassion and rejects the arguments of Corruption.
“And their boast, “Behold, we have slain the Christ Jesus, son of Mary, [who claimed to be] an apostle of God!” However, they did not slay him, and neither did they crucify him, but it only seemed to them [as if it had been] so; and, verily, those who hold conflicting views thereon are indeed confused, having no [real] knowledge thereof, and following mere conjecture. For, of a certainty, they did not slay him: (Quran 4:157)
Quran doesn’t say that Allah allowed to fake the crucifixion so that a large group of people becomes deviated from the true path. Quran simply says that Jesus didn’t die on cross. The story that another man was given likeliness of Jesus is not found In the Quran. Even some Muslim scholars opposed this substitution theory.
Imam Fakhruddin Razi listed the ethical problems and questions with the substitution theory in his Tafsir=clarification such as:
- Since Jesus was aided by the Holy Spirit=Angel Gabriel in Islam, why did he not simply protect himself?
- If God was able to lift Jesus up instantly, what was the advantage of this whole deception?
In this regard, I found Muhammad Assad’s commentary on Quran, most convincing. His comment regarding Quran 4:157-
“Thus, the Qur’ān categorically denies the story of the crucifixion of Jesus. There exist, among Muslims, many fanciful legends telling us that at the last moment God substituted for Jesus a person closely resembling him (according to some accounts, that person was Judas), who was subsequently crucified in his place.
However, none of these legends finds the slightest support in the Qur’ān or in authentic Traditions, and the stories produced in this connection by the classical commentators must be summarily rejected. They represent no more than confused attempts at “harmonizing” the Qur’anic statement that Jesus was not crucified with the graphic description, in the Gospels, of his crucifixion.
The story of the crucifixion as such has been succinctly explained in the Qur’anic arabic phrase wa-lākin shubbiha lahum, which I render as “but it only appeared to them as if it had been so”
– Implying that in the course of time, long after the time of Jesus, a “legend” had somehow grown up (possibly under the then-powerful influence of PAGAN Mithraistic beliefs) to the effect that he had died on the cross in order to atone for the “original sin” with which mankind is allegedly burdened.
And this “legend” became so firmly established among the latter-day followers of Jesus that even his enemies, the Jews, began to believe it – albeit in a derogatory sense (for crucifixion was, in those times, a heinous form of death-penalty reserved for the lowest of criminals).
Allah exalted He did not deceive them; they were deceived by their wrong interpretations and explanation. Such a question is like the question if asked to the believers of Crucifixion.
If Trinity is true, then why there trinitarian Deity would deceive all previous Jewish Prophets to deceive all previous Jewish generations from children of Israel to believe in the original Monotheism belief about God as a One supreme deity?
Allah knows Best.