Why did Diocletian make Nicomedia his main residence?

Diocletian established his main residence in Nicomedia primarily for strategic military reasons. As part of his administrative reorganization of the empire, he sought to maintain a close watch over the East, particularly the Persian frontier, which required constant vigilance and defense.1

By positioning himself in Nicomedia, located in western Anatolia, Diocletian could effectively oversee the eastern regions of the empire, including Thrace, Asia, and Egypt. This move was consistent with the broader strategy of the tetrarchy, where each of the four rulers resided in a different key city—such as Milan, Sirmium, and Trier—to ensure rapid responses to threats across the empire's vast borders.12