No one knows. Its a conspiracy. The conspiracy's not important; FE'ers theorized the existance of the conspiracy to try to examine how people started thinking the world is round and where the false information is coming from.
As for possible motives, I once proposed that it is possible that a round Earth is much more desirable than a flat Earth. In a flat Earth, I believe the universe isn't nearly as big, if there even is a universe outside of Earth. In a round Earth, many more things are possible. Almost infinitely if you think about it. And if you think about it more, this would prove to be highly fruitful if anyone should wish to make a conspiracy on it. Think about it: scientists working forever on something that we could never inch closer on as far as understanding (taking in to account our current knowledge and technology).
I put forth the analogy of Santa Claus. Why do parents tell their children about Santa Claus? There's no one answer. Some parents tell their children out of tradition, to bring joy to their children, to bring something magical to their innocent lives, or because they themselves believe in a spirit of Christmas and wish to simplify their feelings for their children in the shape of a jolly man giving presents. I'm well aware of the last one sounding silly, but I have encountered people who believe this. I also seem to be a sucker for believing in a little magic on Christmas Eve, whether or not it is a significant time of the year.