Answer: Due to geographical relief
Explanation: The Persian empire was very widespread on a large territory because it is such a natural environment, i.e. plains and river valleys, mainly. Even in one part of Persian History, Persian territory was considered the most widespread empire in the then known world.
Unlike Persia, Greece, apart from the land that has many mountains, consists of a large number of islands. This fact is one of the reasons that Greece could not territorially expand its empire, because it is territorially limited by natural relief.
Only if we do not count the great Greek Empire under Alexander the Great, which also contained many parts of other empires. So Greece itself is constrained by its natural environment, so it is naturally not possible to expand on such territory as Persia.