Simply put, one of the reasons optimistic eschatology fell from a place of dominance to occupy the sidelines of the evangelical world was that two world wars and the social-moral upheavals of the 20th century seemed to suggest things really could only get worse. But of course, it's bad news for theology to be based on the newspaper headlines rather than thoughtful, careful examination of the scriptures.
The Jollyblogger thinks postmillenialism might be making a comeback, and that this might be linked to brighter headlines in the US. He (not a post-mill) thinks it would be quite a good thing for postmillenialism to come back, but that any comeback shouldn't be based on the news headlines. I (a post-mill) agree with him on both counts.
Also of note, the article he links to includes an intriguing comment;
These days many evangelicals talk like premillennialists but act like postmillennialists. They expect the world to get worse and worse but preach the gospel, lobby politicians, and fight for social justice in order to make it a better place.