The Great Barrier

Following Moloch marshalling his army on Nexus during his failed coup, the gods grew wary that one day some fool trying to make use of Nexus' prime positioning for otherworldly travel could accidentally free Urthemiel the Destroyer. And so they placed the Great Barrier around Nexus, isolating the world from the much of the multiverse and making direct travel to and from the outer planes impossible. Though it was still possible to reach Nexus by traveling though a series of other planes, coming and going with a simple spell would be impossible. But even for the gods, covering an entire world with such a barrier was no easy task. And not one they were able to execute perfectly.
This barrier was meant to interfere with teleportation magic and exposure to such magic could cause it the weaken. Thus, the gods decreed to their mortal worshippers that teleportation magic was forbidden in order to maintain the barrier and help keep The Destroyer contained. This worked for a good many millennia, until the arrival of the Argus Pirates on Nexus. The otherworldly pirates made free use of the teleportation beam on their spelljammer, the Jeweled Eye, to move themselves around the Ashes Archipelago. Each use of the beam put a hole in the Great Barrier, which began to grow and rip the whole thing apart. By the time anyone noticed the damage, it was too late to do anything about it. The barrier was gone and Nexus was open to would-be invaders from across the multiverse.