It's part of the CDA's mission statement: every human has to ultimately be sent back to their place of origin -- and no other. To simply evacuate all of the incursions to a single receiving world could do as much damage to that place as the intruders are doing to Equestria. But even with some of those realms currently unreachable to ponies, there was always a world on the other side of the Barrier.

One of those worlds is now gone.

Seven billion are dead.

Four survive.

And Bree Daniels, once the CDA's lone agent in the world known to ponies as the abattoir, has nowhere left to go.

(Part of the Canterlot Deportation Agency series, which has its own TVTropes page. Now with author Patreon and Ko-Fi pages.)

As this story is a direct sequel to the original, I strongly[/u] recommend reading that one first.