8
The thing was, people were always coming up with new, secret languages for encoding light. As soon as you had a new one, you could make people fork over a certain sum of cash to see into a given black tower. Once everyone wanted to spend every waking minute in a black tower, it quickly became clear that new towers were needed to operate inside existing towers. Things became particularly confusing when sticky legal questions arose regarding who owned a certain tower's light resources, and who had the right to approve new black towers within old black towers. It seemed that new ones were being built without permission. That's when people began to wonder why everything was being overtaken with limitless void.