If Sombra can casually mind-control ponies, why did his reign see many marching in chains instead? Because only a pony whose mind is “awake” can suffer from being opressed, can feel pain and despair. And Sombra wants that from his subjects - not (only) because he’s more than a bit of a sadist, but because the realm he rules is designed to amplify and project the emotions of the populace. Just as a happy Empire is a beacon of joy across Equestria, a miserable one spreads almost tangible depression.