It was Eden and Zion's first visit to the Milky Way, apparently - though Abbadon was close enough to intersect their course sometime between the planning session and arrival, it may not operate in the same way those two did at all. It may just be that such dimensional walls are only set up for an arbitrarily large physical distance around a target world. As for why it's still there, I can think of a couple of different analogies, but the simplest is that whatever force is actually operating to create the wall is diverted from the empty worlds where the shards are put down so they can reach out and stick a pinky in their hosts' brains, like displaced soil from a worm's hole, and the only input required from the entities is how it was initially formed. When their experiment disturbed the walls of the burrow, either the remaining colony patched it up or it made a "landslide" around where the disturbance came from and that's why the effect is now locally stronger.
When it becomes immediately relevant, I'm sure one of the boffins in-story will explain.
--
"Anko, what you do in your free time is your own choice. Use it wisely. And if you do not use it wisely, make sure you thoroughly enjoy whatever unwise thing you are doing." - HymnOfRagnorok as Orochimaru at SpaceBattles
woot Med. Eng., verb, 1st & 3rd pers. prsnt. sg. know, knows
When it becomes immediately relevant, I'm sure one of the boffins in-story will explain.
--
"Anko, what you do in your free time is your own choice. Use it wisely. And if you do not use it wisely, make sure you thoroughly enjoy whatever unwise thing you are doing." - HymnOfRagnorok as Orochimaru at SpaceBattles
woot Med. Eng., verb, 1st & 3rd pers. prsnt. sg. know, knows