The energy which pokemon require to continuously attack in battles typically comes from powerups provided by their trainers, for only a few, such as Typhlosion, can produce it on their own. These powerups have historically produced in a variety of ways, depending on a pokemon's type; those seeking to manufacture lightning energy once chased thunderstorms, while those who wanted grass need merely gather it from a forest. But the burden on trainers was great and the reward insufficient, and only the nobility tried.
The modern, industrialized era of pokemon battles required the discovery of a few locations in the world where every type of energy could be found – bountiful mines where the air and ground alike are rife with power, and where anyone capable of braving the dangers to harvest it would return home with riches. The mining towns were not particularly exciting places, nor are they today – the businesses which normally spring up after gold rushes saw little profit in such a remote and unsafe location – but, just like everywhere else, they were full of pokemon trainers who would seldom resist the chance for pickup battles in their downtime.
What they discovered in these battles was that their pokemon never ran out of energy; if anyone used a powerup, it would as often as not be completely refilled the moment they dropped it, and if pokemon ever struggled to attack it was only because they couldn't strengthen them fast enough. Some considered the matches in the Energy Stadium a poor facsimile of the ones closer to home, where energy is a scarce resource which must be carefully controlled and never exhausted – but lacking anything better to do, they fought on anyway. Others, however, loved the thought of pokemon matches where they would never experience an energy drought.