I posted before, but I think it is due to the sensor technology.
There have been huge improvements over the past two decades in the capability of the sensors, plus there is now multiwavelength imaging - infra red, optical, radar. The sensors can track automatically, and all the time being recorded in higher and higher definition.
The other issue is that over time, the sensors are only going to get better. The pictures are going to get better resolution, the tracking better and they are going to be able to see stuff more clearly from further away. The ability to deny the existence of something when it is imaged on multiple sensors and recorded becomes that much harder, and it is only going to get more difficult to deny.
Of course, the advances in sensor technology go hand in hand with secrecy. You can understand why the military is reluctant to give away what it is capable of detecting and tracking, because if you know the limits of their equipment, you have a benchmark for designing things that will be able to avoid detection.
So yes there is an issue there that the reluctance to release information may be a "cover up" on the whole alien issue, but may also be because the military are really worried about revealing their capabilities to enemies.