East and West Egg represent, respectively, the split among upper class society of old-monied, aristocratic families and the "nouveau riche" families whose fortunes were recently made. In the time setting for this novel, this division was distinct and very relevant. "Old money" was considered more respectable than "new money" and this is evident in the social depiction of life in East Egg and life in West Egg. Consider, for example, how Daisy (who lives in East Egg) considers Gatsby's (a West Egg resident) parties to be decadent and unlike the civilized gatherings she is accustomed to attending.
Nick ultimately returns west because he has become disillusioned with east coast society. He left the west, like many people of his era, in search of a richer, broader, cultural experience. After his experiences with Gatsby, Nick finds the east and its attendant lifestyle to be contemptible and lacking in authenticity.