The primary reason Diocletian divided the Roman Empire into two sections was that he thought it would make it easier to rule. The Roman Empire had grown too large and complex to be effectively governed by a single ruler, leading to administrative challenges and difficulties in responding to military threats and local governance issues. By dividing the empire into the Eastern and Western Roman Empires, he aimed to improve administration, enhance defense, and maintain stability across the vast territories.
While there were other factors at play in the empire, such as the rise of Christianity and issues related to the Senate, the main focus of Diocletian's reforms was on effective governance and stability.