Their original concept for game balance was to use a move called, "Iceberg", which was basically an ice-type Stealth Rock. However, Game Freak never utilized this, seeing as it would shut down all the 600 Dragons, and Rayquaza would be a joke.
Instead, they thought, "Instead of dumbing down the power of the dragons, let's power up everyone else!". And so, they decided to give Dark, Psychic, Fire, Bug, Poison, and Ghost-types all huge improvements.