Initially they hid Mars being earthlike and inhabited, on the grounds of a report done in the 50's, showing that most of us would go nuts if that was disclosed to the world. And now, they know that they cannot keep this hidden forever, but are delaying it for as long as they can, since a civilization about 1000 years ahead of us, will disclose their free or overunity devices and or their propulsion systems, which leads to antigravity and limitless energy.
Both can be militarized on Earth, and one will lead to free energy and oil stocks plunging, and that is something that the elites of this world cannot allow.
So they release this as a trickle so the foolish can ignore it, and the smart can grasp it, and NASA will quite ironically tell the world when they circle Mars and land on Phobos, (and show HD images of the monolith) probably 2033.