Several factors, in a unique combination, helped set the stage for the development of philosphical thought in ancient Greece.

The ancient Greek economy of the time was based on trade. This made the Greeks quite wealthy and independent. This also put them in contact with many of their near eastern neighbors allowing for exchanges of ideas between cultures.

Also, as Logicon mentioned, the political atmosphere of the Greeks differed from that of their neighbors. There was a movement away from the rule of kings towards more of a democratic structure.

These factors standing alone would not have necessarily facilitated the development of Greek philosophy as we know it, as other cultures shared similar aspects in their societies. However, this particular setting in Greece provided the social and intellectual basis for such development to occur.