Why do some Muslims believe that Jesus didn’t get crucified?
That they said (in boast), “We killed Christ Jesus the son of Mary, the Messenger of Allah”—but they killed him not, nor crucified him, but so it was made to appear to them, and those who differ therein are full of doubts, with no (certain) knowledge, but only conjecture to follow, for of a surety they killed him not:—Nay, Allah raised him up unto Himself; and Allah is Exalted in Power, Wise (Sura 4).
This notion that God somehow spared Jesus and that either someone else was crucified who looked like Jesus or that it was a sort of mirage was not a heresy original to Islam. It can also be found in some early Gnostic writings. For example, the Gnostic Gospel of Basilides claims that when Simon of Cyrene was helping Jesus carry the cross, Jesus switched appearances with Simon, and it was Simon who ended up being crucified. Also, in the Gnostic Apocalypse of Peter, an unnamed substitute is crucified instead of Jesus.
In general, it can be said that Islamic theology finds it unthinkable that God would permit a holy prophet to die in such a manner, and Gnosticism finds it unthinkable that God could die.
As Christians, we understand that Jesus’ death, which was a freely chosen atonement for the sins of humanity, was not the end of Jesus and that his resurrection was the ultimate triumph of God over evil and sin.