Why did Diocletian decide that one emperor could not govern the whole Roman Empire?

Diocletian determined that the Roman Empire was too vast for a single individual to govern effectively. During the 3rd century, the empire faced near-constant instability, with frequent revolts and invasions occurring simultaneously across its expansive frontiers—from Britain to the Persian Gulf and along the Rhine and Danube rivers.12

Because a single emperor could not be present at every crisis point, he was often forced to choose between defending one region while leaving another vulnerable. To address this, Diocletian implemented a system of power-sharing, eventually establishing the tetrarchy. By appointing colleagues to share the burden of administration and military defense, he aimed to restore order, suppress usurpations more rapidly, and ensure that the empire’s vast territories were under constant, localized supervision.2314