Why did the early Fathers hold premillennialism, and how did Augustine change it?

Even as a Protestant I always thought the amillennial position from St. Augustine was correct, and when I became a Catholic I found that Augustine's amillennial eschatology was exactly what the Catholic Church had been teaching all along. What was curious is that our earliest Fathers, some of the big ones like Justin Martyr, Irenaeus, Polycarp, Tertullian, and Papias, were not amillennialists; they were premillennialists. The reason is that the apostles did not give them much on eschatology, and prophecy is written mostly in symbolic language to hide the truth of the future, so it is hard. The early Fathers read the thousand years in Apocalypse 20 literally and placed it at the second coming, teaching that Christ would come to Earth, sit in a chair in Jerusalem, and reign a thousand years before the eternal state. Augustine came along and said no: the thousand years begins at the first coming, not the second, which makes it symbolic time representing the reign of the Church, and that is what the Apocalypse is about. Where you put the thousand years is the most important decision in interpreting the Apocalypse. The basic key is that each of its seven sections runs from the first coming to the second coming. But Augustine kept one piece of baggage from the early Fathers: the idea that there must be a mass conversion of the Jews. The early Fathers had room for it because the Jews would reign with Christ over the Gentiles in the thousand years at the second coming, but Augustine's system, ending with the destruction of the universe at the second coming, leaves no time or place for it. That is where the confusion in Catholic eschatology about a mass conversion of the Jews comes from, and it is about time it was undone.