If crucifixion as a means of capital punishment hadn't died out in the ancient world, it would labeled cruel and unusual punishment, and would never fly in the civilized world. The common wisdom is that the practice ended with the reign of Constantine, the first Christian emperor of Rome (306 to 337 CE). However, some accounts of crucifixion appear after he converted in 312.
Even if Constantine did, in fact, end the practice of crucifixion, it’s not clear that he did so out of respect for Christ’s execution. Aurelius Victor, the earliest historian to claim that Constantine banned crucifixion, explained that the emperor was motivated by a sense of humanity rather than piety. Crucifixion is a pretty gruesome way to go—significantly worse than the New Testament makes it seem. Although Christ reportedly expired in a matter of hours, many crucifixion victims clung to life for days. Even in Roman times, it was considered an exceptionally cruel punishment, reserved mainly for those who challenged state authority, such as insurgents and enemy soldiers. (Joel Marcus of Duke described crucifixion as “parodic exaltation,” because it gave rebels the fame they sought, albeit in a grotesque form.) By some accounts, Constantine replaced crucifixion with hanging, a less painful execution method. Constantine’s supposed ban on crucifixion came as part of a package of reforms, further suggesting that he was merely exercising human mercy. Branding prisoners’ faces, for example, was also prohibited around the same time—a reform that had nothing to do with Christ’s execution.
Read more about the history of crucifixion at Slate. Link