Why was the roman empire divided into two sections?

because Emperor Diocletian decided the empire was too big to handle he divided it into two parts Eastern roman empire and Western roman empire

Explanation:

the conditions that made the byzantine empire vulnerable to attacks by the muslin turks were the sack of constantinople by western crusaders, the conquests of the sultanate of rum, presence of rival kingdoms and finally the set up of fiefdoms at the empire which were owned by the ghazis (turkish warriors).