I will try not to go to far OT.
IMHO the people in power want to stay in power. The knowledge of alien tech will revolutionize society. The introduction of anti-gravity alone may decimate the multi-billion dollar fossil fuel industry. That's just one group of many willing to use their power to obfuscate disclosure to retain their wealth and/or military advantages.
.
But the truth won't stay hidden forever.
On going research will one day lift the veil.