As the world lay dying at her feet, the Silver Queen, in her anguish and despair, knew that for humanity to live, the whole world had to change. Thus it was that Queen Serenity called upon the power of the sacred Silver Crystal, and rewrote the very heart of the world. Jointly, these three wishes are known today as Serenity's Final Decree. Like much about the tale of the Silver Millenium, it is considered apocryphal - a myth, explaining via anecdote, anthropomorphism and just outright fiction why the world today is as it is. Few today believe the Dark Kingdom ever existed, and even fewer put stock in the miracles of the Silver Crystal. But the Decree...the Decree, above those other things, is seen as true, or at the very least the lesson for which the rest of the story exists, for the Decree speaks to the way the world is.
So what did these miracles do?
"Never again shall darkness touch these shores."
The first of Serenity the Elder's three great wishes, which rewrote reality so fundamentally as to render the world before inconceivable to the world after. It was by this wish that Serenity cast darkness from the Earth, and isolated the world from all the many Magical Worlds.
By her first wish, Serenity ripped the masses of darkness out of the Solar System, and cast them far away. Many were flung into the far-off Magical Worlds, Juraihelm and the Garden of Rainbows, the Land of Fountains and Minor Land, and many, many others - and these shut the doors behind them, rendering travel to and from Earth difficult to the point of near impossibility. Some, native to this world, were cast out of Earth's space and ensorcelled to be never again able to find that lost world. It is said that this is the fate that befell the Desert King. Some few, bound inextricably to Earth by nature or pact, remained even in the face of the godhead's wrath...and these few represented a lingering, dangerous threat.
The most important part of this wish is that it rendered the magic worlds almost, but not quite, totally inaccessible. The doors to the magic worlds remained, but they were so difficult to cast open that very few did, and those occurrences were rare. Often only when a magic world was in crisis would the difficult spells be cast - summoning forth guardians, or sending forth messengers. Frequently, it was only the most powerful of worlds capable of such magics - it is said that the Queen of Light and the Pillar of Cephiro are some of the only few in all the many worlds with such staggering might. Some rare few worlds, such as the wizard haven of Mundus Magicus (simply Latin for "the Magic World") have somewhat more ready access to Earth, but even these require gateports and doorways that exist in only a handful of places around the globe.
But there is a twist in this tale. When Clow Reed rocked reality to its core, it was the threads of this miracle he hoped to shred. By his hand, its effects are weakened - darkness has regained much of its former strength, and many more magical worlds are finding it possible to muster the strength to open those doors. It is still not as easy as it was during or before the Silver Millenium, when one could practically pop over to the Kingdom of Sweets for lunch, but far more often are refugees from magic worlds being found on Earth - and far more often are the forces of darkness slipping through the cracks to menace the world.