In answer to why the paradigm shift exists, I would ask why any paradigm shift take place? The answer is probably that people get bored of the same thing, new thinkers arise to resurrect old stuff, and the cycle continues. Mood is a determining factor with Christian eschatology, too. People look around to see how good or bad the world is and then decide which view makes sense in light of that. Prior to WWI people tended to opt for postmillennial positions due to their optimism. After the Great War people turned pessimistic and consequently became premillennial. During the 1970's and 80's Evangelicals felt more in control politically, so the language used to discuss the already/not yet phenomenon took on a more postmillennial tone once again. Another factor responsible for eschatological positions is one's theological background (at least in denominations where the founder's position continued due to sociological reinforcement and cohesion). I think here of Lutheranism which has mostly been Amillennial from the start, and the Reformed and Presbyterian churches, many of which have always been Amillennial. The introduction of Bible colleges brings another factor to the table. Many colleges and seminaries indoctrinate. The school you graduated from may have taught you an eschatological position and told you why that one was the correct one! John Walvoord did much to strengthen Dispensationalism and Premillennialism at Dallas Seminary, to provide one such example. Finally, it is important to realize that Christian ideas go through cycles like all others.
