Less Population Density

• The population of ancient Europe was less dense that it is today. A relatively small group of well-armed soldiers could attack and conquer large swaths of land because they met little resistance. There was the occasional town and scattered farmhouses, but there were few large cities capable of defending themselves. With today's population, many more people would be fighting back and many more cities would defend their land and people.
  
  

Communnication

• When the Greeks or Romans would attack a village or city, the only way for another village to know was for a survivor to come and tell them about it. The surrounding cities had little time to prepare for an attack since there weren't may ways for them to know about the coming army. With today's fast communications technology, the cities would quickly know enemy position and strength.
  
  

Education

• A significant education gap separated the ruling empires and the various cultures they conquered. The Greeks would attack with fire and other devices while lesser civilizations could only attack with crude spears and bows. They did not have the military knowledge and experience of the ancient empires and were often easily conquered.
  
  

Superstition

• The world of the ancient empires was mostly unexplored, so civilizations developed and grew with little knowledge of each other. It wasn't until an empire decided to expand or explore that the civilizations knew of each other. In cases such as Cortez and the Aztecs, he played on their superstitions as a "White God." In other instances, the superior technology of the invading force was looked upon as magic by the smaller civilization.