If I'm recalling properly, in the first movie the whole reason everyone except Ripley dies is because of the automaton with override instructions to bring home the alien specimen intact. Furthermore, it would seem that they were already thinking of warlike purposes for any such entity they might be able to get their hands on.

When you think about it, you could then assume that the government (as Antigone suggested) was atleast aware of a possible alien creature with a capability for destruction far beyond what humans could achieve without the means of physical weapons and what not.

So yeah, it is entirely possible that this could be an information held close sort of scenario or even again as Antigone suggested just a case of Ripley was the first recorded survivor and we get to see every human erased from a movie by the creature from space (always a good ending).