Why do you think the leaders who followed the Macedonian emperors were unable to maintain the gains and prosperity they had achieved?

There are multiple reasons for this but the most significant are:

- They were fighting against each other constantly; the leaders that followed after the creation of the enormous Macedonian Empire that was stretching on three continents, divided the empire into few parts, and instead of maybe acting like a confederation so that they can preserve the strength, wealth, and prosper of the empire, they were waging wars against each other and weakened all of the parts of the empire that they were ruling over, which in the end led to the Romans being able to take over the territories in Africa, Europe, Asian Minor and parts of the Middle East.

- The territory left from the previous emperors was too big for a single ruler; the territory left by Alexander of Macedon (the Great) was enormous, and it included lots of different ethnicity, religions, languages, cultures, and also there were lots of people that wanted to be out of the Macedonian Empire, so dividing it into smaller parts so that better and more efficient ruling can be implemented seemed logical, but unfortunately it turned out to just backfire on the leaders that were ruling the parts of the Empire.