Answer:
I believe that since people back then, were less able to afford housing than they are now, buildings were pretty much a big deal for them.
Not to mention, around this time new models of homes were also being built, and so people were moving around a lot more (meaning that demand for houses would be higher).
The U.S most likely kept some images classified, as to not worry or alarm the public, and to protect the public images/reputations and privacy of the people who may have been involved in the related incidents. They also probably waited until the 1980's, because that's more or less around the time that other major events were taking place, thus people were already getting fed up with the government's lies and so would force an explanation from them. Of course, since the government doesn't want to make enemies with its own people, it would be forced to release its information, but only because the people asked for it.
Explanation:
Hope this helps, I tried :)
Sorry if it's not what you were looking for though.
