What was the cause of the difficulty in uniting Ancient Greece?

Geography always plays a very important role in how humans organize their culture.  This was certainly the case with the Ancient Greeks as geography made it nearly impossible for the early Greeks to unify under a single government. The landscape of Greece is very rugged and mountainous and travel between different areas of the peninsula was very difficult in ancient times.  As a result, regionalism was the reality of those living on the mountainous peninsula.  Each region developed into an independent city-state with elements of government that were unique.  Some city-states, like Athens, evolved into governments that allowed their citizens a greater role in governance.  Other city-states instituted oligarchies or retained monarchs.  With the variety of government types that developed as a result of the rugged geography, uniting Greece under a central government was a challenge that would not be met until the conquests of Alexander of Macedon in the Fourth Century BC. 
http://www.ancientgreece.com/s/Geography/