I have been studying ufo and aliens for years now and I am aware of the cold war climate under which is the beginning of the US government really hiding things.
The other reasons in brief are.
1. the government would have to openly admit they can,t control ufos.
2. the ufo power systems could be used to blow us all up.
3. the ufo power systems will kill the oil,gas ,and car industry.
4. It might show there is a different god than our taught religion does.
5. It will change the world power structure.
(These and a few other things are what I understand to be the reasons for secrecy ).
Here is my exact question. I have heard that many of the ufo,s contain mostly examining equipment. I have also heard it is very important to the aliens that they keep abducting man. I think if the general public were to find out our government has been letting the aliens abduct us we might put a stop to it. This force the aliens to openly harvest us and it might cause a war.