I forget the name of the new series with the ex .gov. guys doing the investigating, but it is very compelling.look no farther then the 1960's era Brookings Institute report that says, we cannot handle the truth and any revelations should be trickled out after preparing the public for the news.
Back then the religious side of life weighed heavily on the decision to keep things secret. We have moved away from religion so those considerations do not have as big an impact today as it would have 50 years ago.
Time travel is the most plausible explanation, but if we believe the Ancient Alien theories there are more than one species at work here.