Why did Emperor Diocletian make Milan an administrative centre for the western empire?

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People walk and shop inside the ornate Galleria Vittorio Emanuele II in Milan, Italy, with its glass-domed roof, decorative mosaics, and luxury store entrances. — Milan Shoppers in the Galleria Vittorio Emanuele II, Milan.

Emperor Diocletian designated Milan as an administrative centre for the Western Roman Empire primarily to enhance the empire's military defense.1

Recognizing that the vast territory was too difficult for a single ruler to manage, Diocletian implemented a system of shared power. He appointed Maximian as his colleague to oversee the West and installed him in Milan. This strategic location allowed the administration to effectively monitor and prevent invasions by Germanic tribes. By establishing regional residences, Diocletian sought to restore civil order and ensure that military leaders were positioned to respond rapidly to threats across the empire's extensive frontiers.1