Although the provisional designation of 2005 FY9 was given to Makemake when its discovery was made public, before that Dr. Brown's team had used the playful codename "Easter Bunny" for this small world, because of its discovery shortly after Easter.
There was a time when Earth had no Moon. About 4.5 billion years ago, when our ancient Solar System was still forming, the dark night sky above our primordial planet was moonless. At this time, the Earth was about 60 percent formed, although it did have a differentiated crust, mantle, and core. This was a very chaotic and violent era in our Solar System's past, with planets first forming out of blobs of primordial dust, gas, and rock. During this era, frequently likened to a "cosmic shooting gallery", collisions between the still-forming planets were commonplace. Orbits were not as orderly as they are now.
Second, there is the issue of sharing wealth. The Third World wants to redistribute wealth in its favor, and it pursues this end by a combination of moral persuasion and threats of terrorism using weapons of mass destruction. A frontier could make such threats less persuasive. The Third World reaction to a space frontier initiative is unpredictable and possibly violent. This makes pioneering a taboo for Western governments. And that's why they would keep it secret.