Why a cover-up for so long?

I asked myself this question tonight after watching a couple UFO documentaries. Why would the Govt. cover up UFO's for this many years? And I think one of the best reasons would be because if intelligent life from other planets does exist, how would our Govt. come out and tell us? I truly think that if they told us that there is intelligent life on other planets they would realize that Americans would ask the simple question...why the lie for so many years and how are we as Americans to trust the Govt. after such a huge lie? How would we trust the Govt. in choosing our wars or sending our troops overseas after that information? Trust is HUGE. Perhaps they are afraid of some sort of retaliation by angry Americans or religious freaks after the announcement? Seriously if they thought confidence and trust in our Govt. is lacking right now, just think what would happen if they dropped that bombshell. That's why I agree with the theory of strategic de-sensitizing of Americans through UFO documentaries,etc. Also lately there is great interest in the scientific community about life on Mars, and I believe that if it is determined that there was or is life on mars, that will be the major turning point for the UFO community and the rest of the world and thus the "bombshell" will be dropped very soon afterwards. Comments?
M@

Think of it this way...

YOU are the government.

YOU are tasked with protecting your citizens, and you are put in place by those same citizens. Indeed, the entire infrastructure of the government is providing certain basic needs to the citizens, one of which is defense.

YOU then are faced with unidentified craft invading YOUR airspace at will, near some of your most secret installations, and exhibiting characteristics far and above any known terrestrial craft.

YOU have no idea what these beings' goals are, etc. and you can only assume they pose a defense risk.

YOU then find out they are abducting citizens, but attempting to wipe the event from their memories...

YOU still haven't got a clue how the crafts work, or what their agenda is...

Why WOULD you then tell the citizens? No, you'd make damn sure this was kept quiet until you could at least figure out the basics. And, what if you then learned those answers....and then didn't like them? Yep, it'd STILL be kept quiet....