My thoughts are that it might be a consensus between the highest echelons of powers that ayy's existence brought to the public would cause a very harmful situation to either their interests or the interest of society as a whole.
If we had open access to ayy's, probably people would ask for answers for all the mysteries of life, from being to why the sky is blue, and in turn they might be worshiped as gods (even if they claim that they aren't or that they serve god). That in turn might bring instability to the States that stray from the teaching of ayy's, with people demanding that we follow their system of government or that maybe they pick our leaders, since they might be able to see the most capable of us. Cults that worship them will pop everywhere, and maybe some give up on their material possessions and social rank just to follow them almost as Buddhist monks. On the other hand, religions will be put on public scrutiny, and many will wonder why the teachings of their prophets aren't like those of the ayy's in certain regards, which is bring them down as an institution of social control, no one will believe the Pope or any religious figure no longer.
I mean, this is just one case, out of many that I'm not willing to write about because of time, but you get the idea.
Those that get to decide whether or not this is public knowledge have decided that it's better to keep it secret because it will harm them or everyone. I mean, people might legitimately give up on consumerism just to follow the teachings or social system of a more advanced society, which will make all our contemporary civilization crumble.
Also, think about it from the ayy's perspective, they probably have experience on this stuff, they might have already visited multiple dimensions with different approaches, and they should have decided that we are not yet ready to know this stuff, so they told governments to keep quiet and to not cause too much hassle.
Also, there is the chance that ayy's are not so benevolent and in order to get the maximum benefit out of us, they need to act in secret.