If it were a joke would we tell you, or just continue laughing at all the people who take it seriously?
If we knew why the government were keeping the reason for the round earth lie secret it wouldn't be a secret. Of course we don't know it's the "government" who are part of the conspiracy. Obviously some parts of some governments must be in on it but there could be plenty of people in the government who remain in the dark.
If the earth is in fact flat, then I think the people saying it is round would be the insane ones, I could be wrong though. There was a saying that whenever there is some kind of movement, the majority is wrong. For example slavery, but on to other things.
Ok, so you don't know why they would keep this secret. You just know they do. Maybe you could think of listing all of the reasons you think they would. It might draw more people's attention. One said it was for money, that made no since whatsoever..... I think that'd be the only reason why anyone would think about making a lie this big. That, or they are hiding something on one of the edges of the earth.
Maybe it's a organization similar to the U.N. only they are planning on killing everyone to start a new organization. If this was true, I think it would be possible for them to keep this secret without telling everyone the earth was flat. That, and if it was something like this, the plan has been going on for a long time.
Then, there are movies like "March of the Penguins," was that whole thing just staged? Maybe someone should go down to Antartica until the very edge of the earth, and film it. Unless there are lots of troopers down there guarding it for some reason. If that's the case, that would be a huge question still going on today.