Travelers to Hoenn will often note the paucity of outdoor lights in the region. Streetlights are rare, as are outdoor lights on houses, and some trainers will even recommend making their pokemon use Flash in cities for self-defense: stargazers flock there because it is much easier to see the night skies.
When outdoor lighting was first installed in Hoenn, lights judged safe by neighboring regions, an epidemic of Dustox poisoning spread with every installment of light. Dustox are a vengeful pokemon who spread their venom with every flap of their wings; a slow-acting but painful poison which can be fatal in large quantities, but is usually only concentrated enough to even be noticed when a Dustox is trailing someone to avenge harm it suffered as a Cascoon.
Dustox, however, are also inexorably drawn to certain colors of light. Night after night the Dustox congregated around the streets of Hoenn's cities, and as lighting expanded a mysterious new pandemic decimated Hoenn's population. Modernity itself was blamed, and desperate mobs of luddites smashed lights, potions, and anything else thought to be disapproved of by the gods. By the time a young physician realized that the strange new disease was not caused by some strange new virus or bacteria, but by Dustox poisoning, and could be addressed with nothing more than bug repellent around lights, every light in Hoenn had already been smashed and a government fearing revolution if they did not act had sought to ban even the stars.
The laws which darkened Hoenn are gone now with the passage of time, and lights surrounded by bug spray were eventually erected on a few city streets many decades later. Yet though the laws passed in its aftermath are gone, this painful historical memory still darkens Hoenn to this day.