In most of the world, above-ground power lines are used as the means of delivering electricity to the vast majority of homes. This was also attempted in Sinnoh, but the project was soon abandoned at great expense, because Pachirisu would run along the lines as though they were tree branches. Their electricity would meld with that already in the power lines, often causing surges and frying equipment; at times the overload would create explosions which snapped the lines outright. The engineers of this electrification project had come from Kanto, and their security measures had been designed to prevent Pikachu from gnawing through the lines; against an arboreal pokemon like Pachirisu, it was useless.
So Sinnoh decided to bury its power lines. Ground pokemon are immune to electricity, and the lines are sturdy enough to resist their daily activities and even most earthquakes, so again theur greatest danger has been the Pachirisu. For Pachirisu are also known to dig deep into the ground in order to bury apricorns and berries for the winter, and it remains quite difficult for even modern power lines to handle direct contact with these electric pokemon.
Before widespread electrification, Pachirisu were seen as just another common pokemon. Today, Pachirisu are one of Sinnoh's most despised pests, and are still probably responsible for the majority of the region's power outages. They were hunted to rarity, but not to extinction, and their survival in the wild owes much credit to the efforts of the Sunyshore Gym. Volkner and his predecessors are unwilling to allow one of Sinnoh's electric pokemon to be driven to extinction, and they have used their leverage with the power company, Sunyshore being the biggest consumer of electricity in all Sinnoh, to force them to stop killing Pachirisu.