The Sundering

My Dear Wormwood,
I wonder you should ask me whether it is essential to keep the patient in ignorance of your own existence. That question, at least for the present phase of the struggle, has been answered for us by the High Command. Our policy, for the moment, is to conceal ourselves. Of course this has not always been so. We are really faced with a cruel dilemma. When the humans disbelieve in our existence we lose all the pleasing results of direct terrorism and we make no magicians. On the other hand, when they believe in us, we cannot make them materialists and sceptics. At least, not yet.
— The Screwtape Letters, letter VII, C. S. Lewis

He wants them to learn to walk and must therefore take away His hand; and if only the will to walk is really there He is pleased even with their stumbles. Do not be deceived, Wormwood. Our cause is never more in danger, than when a human, no longer desiring, but intending, to do our Enemy's will, looks round upon a universe from which every trace of Him seems to have vanished, and asks why he has been forsaken, and still obeys.
— The Screwtape Letters, letter VIII, C. S. Lewis

Clearly, supernatural activity does not come to our daily public attention. In fact, it is very hard to bring anything supernatural to public attention, as anyone involved in supernatural doings quickly discovers. Contrariwise, it is very easy to hide the supernatural from ordinary, mundane attention. This twist of luck or fate separating the esoteric and prosaic worlds is called "the Sundering." It is not, however, leak-proof or air-tight. After all, myths and legends do circulate in the prosaic.

The Sundering varies in force from place to place. It tends to be weak in isolated areas with small populations. Such places are not likely to blow the cover to the whole world.
The Sundering varies in force over time. When magic is already believed in anyway, a little show of more magic isn't going to make much difference.
Even in high populations of highly skeptical cultures, individuals and small groups can be exposed to bizarre phenomena.
Once those individuals are exposed, they have a good chance of becoming "Sundered," of going over to the esoteric side themselves, if they stubbornly pursue matters. That means they will readily run into esoteric events – the Sundering will no longer shield them – but they will now have a hard time bringing such events to public attention.
Given the kind of things that go on in the esoteric world – supernaturally powered conspiracies to control the fate of nations, predation by vampires and other monsters, occult traffic in souls or prana or geases, wizards and mad scientists with insane agendas – a newly-Sundered individual may not last long, especially without help.

But why is the Sundering there? Why shouldn't esoteric stuff be exoteric, public, as depicted in so many fantasy novels? Frankly, no one knows. There are many theories:

Heaven and Hell may have differing but parallel moral reasons to impose the Sundering, as in the preceeding quotes from The Screwtape Letters.
Heaven may impose the Sundering to protect the prosaic world from the esoteric.
Heaven or Hell or both may impose the Sundering to protect the esoteric world from the prosaic.
Heaven and Hell may want their war to be a cold war at the present time, and impose the Sundering to keep it from heating up.
It may be that some great but prana-starved thing works to prevent too much prana from being expended in magic, and, to keep magic rare, keeps it secret. In other words, maybe one or more of the old gods or some other myth-creatures are saving up prana to make a come-back.
Someone once made a really big wish. No one has any sensible idea who made or granted this wish, but many have tried to guess the terms of the wish. They were, perhaps, along the lines of "I wish magic would never bother ordinary people again." The point here is that, if ordinary people go seeking magic, the wish does not apply. It is also important to remember that the performance of a wish is limited by the powers of the wish founder.
It may be a timelock; that is, the Sundering may exist because the present age has been affected by some event in the future, which would not come to pass in a world without a Sundering. Example:
At some point in the future, the Antichrist rises to power. He could not rise if myth and magic were public. Too many seers. Too many armies of superheroes. In short, the ordinary world would be too powerful for him.
But we have already been affected by his rise, through the prophecies of Revelation. It doesn't matter if the prophecies remain believed or understood or even remembered. It only matters that they were heard and had some effect, any effect, on history.
° We would not have heard the prophecies if the Antichrist will never arise.
° The Antichrist will never arise if magic is public.
° Therefore, since we have heard the prophecies, magic cannot be public.
Now, this does not exclude the idea that the Sundering is caused by Heaven. Heaven may have sent John the prophecy in order to create the Sundering, thus protecting many generations of humanity from widespread exposure to magic and setting up human pride for its own comeuppance in the form of the Antichrist, who, like Babylon or Egypt, is both foe and punishment.
If you don't like the prophecy of the Antichrist, you can substitute the prophecies concerning Ragnarok, or the Kali Yuga, or the works of Nostradamus.
But the temporal dynamics are the same, whichever prophecy and whatever the higher-level cause of the Sundering. It is an impersonal effect, warping probability just as much as it needs to, to achieve the result.
It could be a part or side-effect of a general trend of diversification in the universe, the prosaic and the esoteric drawing apart along with other categories:
      "Have you ever noticed," said Dimble, "that the universe, and every little bit of the universe, is always hardening and narrowing and coming to a point?"
      His wife waited as those wait who know by long experience the mental processes of the person who is talking to them.
      "I mean this," said Dimble in answer to the question she had not asked. "If you dip into any college, or school, or parish, or family – anything you like – at a given point in its history, you always find that there was a time before that point when there was more elbow room and contrasts weren't quite so sharp; and that there's going to be a time after that point when there is even less room for indecision and choices are even more momentous. Good is always getting better and bad is always getting worse: the possibilities of even apparent neutrality are always diminishing. The whole thing is sorting itself out all the time, coming to a point, getting sharper and harder. Like in the poem about Heaven and Hell eating into merry Middle Earth from opposite sides ... how does it go? Something about 'eat every day' ... 'till all is somethinged away'. It can't be eaten, that wouldn't scan. My memory has failed dreadfully these last few years. Do you know the bit, Margery?"
      "What you were saying reminded me more of the bit in the Bible about the winnowing fan. Separating the wheat and the chaff. Or like Browning's line:
      "'Life's business being just the terrible choice.'"
      "Exactly! Perhaps the whole time-process means just that and nothing else. But it's not only in questions of moral choice. Everything is getting more itself and more different from everything else all the time. Evolution means species getting less and less like one another. Minds get more and more spiritual, matter more and more material. Even in literature, poetry and prose draw further and further apart."
— That Hideous Strength, C. S. Lewis
It could be a combination of the above.