As you can see, not many people can handle the impact of knowing
what lies out there. Should government officially disclose and
acknowledge the presence of extra-terrestrial lifeforms, especially
intelligent ones - it might reasonably cause an abrupt breakdown of
socio-economical structure we have progressively and painstakingly
build up. We're talking about the how religions, economy, society
and everything else will abruptly change should such secret be
disclosed.
Another valid reason is the fact that Earth consist of several
sovereign countries and each have their own way of governance and
policies - and clashes of such differences result in conflicts. No
matter what, technically while there isn't any tangible wars,
political wars are still being fought and disclosing alien
technologies is the last thing a sovereign country would do.
Possible negotiation or contact with intelligent lifeforms and
acquiring of alien technologies from them are kept confidential and
being held as the "trump card" should war broke out.
Like the old sayings goes: Some things are better kept secret.
