The photo above shows a serapeum from Egypt, which has been closed to the public for over 50 years.
For blocking access to the most interesting ancient objects, I see only a few logical reasons, which I will now tell you about.
The most obvious and simple reasons, I believe, are the fear for the safety of the object and the difficulty of archaeological work at the site.
If you open access to tourists to the object, which is still being excavated in full, they will greatly interfere, and there is still a chance of damage to ancient monuments. However, there is another side to the coin.
Some zones can hide not only for several years, but also for many decades. In this case, I have logical suspicions.
What if they contain artifacts that contradict the official versions of history? Smaller finds can be quietly taken out of the premises and opened for tourists. But what about the finds that cannot be easily removed?
For example, the sarcophagi from the serapeum weigh about one hundred tons, so it was not possible to take them out. Well, imagine if a temple from an alloy unknown to science was found in some underground complex?
How to get tourists there if alternative history researchers instantly spread the news all over the Internet? In general, there can be a lot of such finds and they may not seem so obvious. Let's look at the example of Egypt.
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In our time, Egyptologists still confidently declare the construction of the pyramids by the ancient Egyptians, although many facts suggest otherwise. Since no ancient references to their construction have been found, this gives rise to speculation with any versions in favor of the government's benefits.
But what if in some distant temple they find drawings of how these pyramids were built by some aliens on plates? I suppose if this place is only excavated, then the general public may not know about it at all. There will be no mention of him in the media, and the area around him will be immediately closed for visits.
Brief conclusion
Let's summarize briefly. In my opinion, governments prohibit access to ancient sites to avoid damage to them and create unnecessary hindrance to archaeologists, as well as to hide more interesting and mysterious finds and artifacts.