Long ago, the more animalistic aspects of Equine nature prevented Equestria from developing beyond the most basic aspects of civilization, dominated by their instincts and needs. However, near the dawn of civilization, the driving magical force behind it was sealed away, allowing culture and civilization to grow beyond that of mere animals. And the magic that rode them was sealed away inside an ancient device, never to be disturbed or it would unleash that magical impulse once more.
...they really should have put it somewhere safer.