The mundanes dun it.
Now, that's a real easy one to motivate. Leaving aside the perils that such powers put them in, they could also easily do it out of spite. Or hopes that no longer trusting on magic would allow them to find their own way.
The bear, of course, would be their having a way to do it.
It could blur into the magicals being the ones to create a magical barrier -- especially if it has to be guarded and protected from breakage. Once a long, long, long time ago, a mundane did the magical king a great favor -- or tricked him into a boon -- and the price was that the magical beings had to erect and maintain a magical force that would separate them.
For the mundanes to have done it themselves would require their -- hmm -- learning the magical weakness. Perhaps they put significant items -- rowan or iron -- in significant magical locations to bind the magical beings. Or perhaps they just popularized something to keep them off. Say, iron. (Which means no urban fantasy. Magic would have to live in the country, or the wild, to avoid the iron.)