Our present government is no more willing to make the move for E.T. Disclosure than our past leaders were to let African-Americans and women vote, or to recognize gays. Enough people must support the issue until it becomes a powerful voting block. Then politicians who must get reelected begin to listen.
The reason by the forces that control the cover-up might be one (or a combination) of the following.
Complexity: The ET presence is not a simple story such as "they are here" as TBar says. All collected evidence points to the fact that the final story will be very complex and possibly multi-dimensional. The more researchers study the phenomena the more visiting races there appear to be, and the more complex their technology looks. It appears to be far beyond what we know or can even comprehend.
National Security: Jimmy Carter entered the White House determined to release the UFO files, inspired by the fact he himself had had a sighting. However, after a UFO briefing Carter changed teams joining the cover-up side. He told actress Shirley MacLaine"it was true…there were occupants," and that he wanted to "shine the sunshine laws on it to see how the people would react," but he "couldn't and wouldn't."
Past Statements: The government has lied for almost 70 years and thus must continue the lie. One lie has been used to cover another and it is becoming impossible to come clean. The government must first come up with a way to explain they have been misleading the public for 70 years.
Collapse in Belief Systems: This would be particularly evident among government scientists and engineers who would have to deal with the fact that their employer has been lying to them and feeding them false data, and more importantly the fact that most of their treasured beliefs in science we wrong.
The 1961 Brooking Institute study looked at this situation.
""It has been speculated that of all groups, scientists and engineers might be the most devastated by the discovery of relatively superior creatures, since these professions are most clearly associated with the mastery of nature, rather than with the understanding and expression of man."
As science fuels the modern technological advances, and upheaval would negatively affect the economy.