Why does Gatsby buy his house in West Egg?
In the novel, both East Egg and West Egg are wealthy neighborhoods. However, there is a difference in the wealth. Those who live in East Egg, such as Tom and Daisy, come from old money that has been in their families for generations. Those families are independently wealthy and have typically have been established in East Egg for several generations and their homes are left to them or given to them.
West Egg's wealth is new money like Gatsby's. The houses may even be bigger than in East Egg, but they are seen as garish and tacky and less refined than in those in East Egg. Those in West Egg still work for their money or they have come into their wealth through the booming stock market of the 1920s.
Symbolically, West Egg is as close to Daisy as he can get. He has a view of her home across the water from his dock, and he yearns towards her and her green light. This distant view of the precious Daisy (in his eyes) is as close as he gets.