What were some of the geographical and political reasons which explain how Alexander was able to assemble such a vast empire in such a short time and why his successors were unable to hold it together?

Alexander the Great

My essay prompt is:  What were some of the geographical and political reasons which explain how Alexander was able to assemble such a vast empire in such a short time and why his successors were unable to hold it together? Be sure to provide specific points to support your argument.