Many things to consider here...
1. Obviously, the aliens want to keep the secret too, as it's entirely up to them if they want to reveal themselves or not. They would not need
governmental cooperation for this.
2. If their purposes are sinister, it certainly makes sense to hide this.
3. If their purposes are benign, the impact must be monitored and meted...
Think of it this way. Let's say the aliens introduced just ONE advance...a clean burning engine that requires no fossil fuels, etc. Just this one
introduction has the potential to completely collapse the world economy. Car sales halt, as consumers wait for the new engines. Gas stations go
bust, the oil companies go under, the airlines struggle to retool...all with millions of unemployed people with no marketable skills (as the skills
are now obsolete). A similar thing happened to the scribes of Europe after the printing press...but think of this on a much larger and more global
scale. With economies collapsing, wars for resources wage, etc. This ONE little thing suddenly brings about the apocolypse... Now, imagine just a
handfull of such advances, suddenly realized.
4. What if their purposes have yet to be stated, and any meeting between them and government has been guarded at best? Without knowing their agenda,
would YOU go public, as a government official?
5. The aliens have shown a keen interest in defense installations. Most early sightings were all around such sites. There are even cases of direct
interference with operations. Surely, this isn't something you'd want the people to know, that there are intruders in your airspace, you're
charged with protecting them, yet there's nothing you can do about it. Not exactly something you want to say at a press conference.
To be perfectly honest, as interested as I am in the topic, even I wouldn't just support total disclosure. I think it has to be done in stages...but
much faster than we're seeing. Something like this....
1. Announce the find of water elsewhere....(NASA has pretty much done this already)
2. Announce the existence of extra-terrestrial microbes (kind of did this, but then suggested terrestrial contamination possibilities)...
3. Announce the discovery of extra-terrestrial insect life (as the majority of species on Earth (more than all others combined), it's likely most ETs
are insectoid, if we apply our ideas to other worlds). This would likely be fossilized, such as in meteorites, etc. or on other planets by robotic
4. After the announcement of such complex life forms on other worlds, the public will be better equipped to deal with the idea of sentient life
elsewhere being not only a possibility, but a likely one. In addition, some of the newer ideas in physics will have to be "dumbed down" and made
palatable to the masses in order to understand the possibility of interstellar travel.
5. The media would be brought on board, to stop ridiculing the topic, and these previous steps would go a long way towards this.
Then, you'd be in a good position to present aliens...but of course, only if their motivations were not unpleasant. I sometimes think the best
evidence for a malicious motive, is the fact that neither they, nor the government, has let the cat out of the bag yet..