There are a handful of historical reasons why after three hundred years the Church overall shifted away from premillennialism to amillennialism. Historian Richard Kyle encapsulates one of the most significant reasons:
“The rejection of apocalyptic [pre]millennialism [by later church fathers] must be seen in its proper context. Constantine had been converted to Christianity in 312, thus ending the persecution of Christians. The hope for the imminent* return of Christ remained strong as long as Christians were a persecuted minority. But when Christianity became the official religion in the Roman Empire during the fourth century, these [pre]millennial aspirations either declined or took new forms” [particularly amillennialism, which was promulgated by Augustine’s teaching] (The Last Days are Here Again Grand Rapids, Michigan: Baker Books, 1998, pp. 20, 38-39)
(*By “imminent,” the early Church did not believe in what is popularly known today as the pretribulational sense of the term. That is, pretribulationists believe that Christ’s return cannot be imminent if prophesied events must occur before the rapture. The early Church held to the opposite (and Biblical) sense of the term in that the rapture was only imminent when certain events must occur first, such as the rise of Antichrist and the Great Tribulation—and that is exactly the Prewrath position.)
Click here to download a PDF for an excellent article, which unpacks other historical reasons for the emergence of Amillennialism a few centuries after Christ. It is titled, “Amillennialism: Examining its ‘Origens'” by Gary Vaterlaus. In fact, send the PDF link to your Amillennial friends! I’d liked to really see Vaterlaus’ article circulate out there. His article does not deserve to be packed away in the dusty archives.