Although today they are explained as Vigoroth running out of energy, this is an area where religion contradicts folk explanations. According to scripture, Slaking became lazy because Arceus sealed their power, for they possessed the strength to overthrow the gods. Able to beat ordinary pokemon easily, they challenged the gods to battle after battle. Although each Slaking won as many matches as they lost, there were far more Slaking than gods; before long, the gods retreated to the heavens. Now unopposed, the Slaking proceeded to shape the world with their massive power as the gods had before them.
The story of the Slaking has two contradictory endings which theologians feud over and apologists try in vain to explain away. The first is that Slaking were an accident, a pokemon to whom Arceus had given too much power. When they created nothing but chaos beyond chaos, Arceus decided to seal their power. This is proof that Arceus is not omniscient; Arceus had created a species too powerful for Arceus' own designs and sealed their energy to correct that error.
The second school of thought ascribes to Arceus omniscience of all except the divine, but not omnipotence, merely an enormous creative power. In this view, Slaking were intended to dominate the earth as the gods dominated the heavens. Yet the other gods were protective of their own power over the earth, and loathed the Slaking for treating them as nothing more than strong pokemon, so they revolted and forced Arceus to seal their power. Arceus, forbidden from creating life which could physically best the gods, yet determined to grant mortal life the power to rule the earth, created a creature whose power was not dependent on raw strength, but on intelligence, language, and the opposable thumb. And Arceus called it Man.