In any government, interests precede truth.

For the sake of popularity, vote, and reelection, most governments remain secretive to the public about the truth of alien contact with mankind. For them, it is better like that than being thrown from the throne.

You govern people, you do good and bad things.
If you don't have guts to do bad, then step aside.

Let us think for a moment about alien, regarding possibility of the existence of other civilization beyond our dear planet earth. We probably conclude that it's all just hoax, or some rumors issued by ruling government for some purpose. Now, we can contemplate a bit about it by asking this simple rhetorical question, 'How could possibly people be able to think about something unknown, which have never been popped out in human minds at all?'

A ruling government can't afford to publish history in a version which lessens its' chance for reelection.

Which one is the truth, sir?
Which period do you mean, son?