I have this theory that in the Kanto region there was a nuclear war or some other event that had to do with nuclear energy/radioactivity, and that it heavily affected the Pokemon and even some inanimate objects there. This is why there are so many Poison types, Pokemon based off inanimate objects (like Voltorb), and Pokemon that evolve into copies of themselves (like Diglett/Dugtrio and Magnemite/Magneton); they were all affected by the radioactivity.