i know why earth hasn't contacted the GTVA yet. because when the lucifer popped out of subspace....well the front half anyway.....the radiation released from her reactors screwed the earth up....now it's that horrible planet of the apes. ok.......
next theory.......
we all know that shivans can travel through subspace without nodes, or at least that is beleived by some. if you subscribe to this theory......the shivans have been at terra this whole time, wondering makes us tick. After all, they had already beaten the vasudans, they wanted to know what finally defeated them. And they were finding out. Communications have been cut off by the shivans.
next theory......
if you do not subscribe to the "shivans don't need portals" theory, then perhaps an explosion large enough to kill a natural portal disrupted the em balance of the entire system.....remember that in the early 80's it appeared that the planets were going to line up, and the gravitational forces would rip them all apart. or so the hypothesis went.
If we expand on this just a little, the position of the planets in relation to one another, coupled with that event (detonating subspace node) created some new anomolies, or at the very least trashed all of the EM...acting like the EM pulse of a giant nuke, capable of disrupting an entire star system. With an EM pulse of that magnitude, we would be looking at a world that has thrown itself into barbarism, because it was suddenly without all of it's technology.
Why didn't this happen in the other systems when we closed portals purposely? That's easy. The planets of our solar system were aligned in such a way that it could happen. This however was a fluke. And will probably not happen again in the next million years. In any system.
last theory......
i am full of crap, and this has all been an exercise in imagination.