When Nick returns from the war, why does he decide to go East in The Great Gatsby?

As the previous post says, ostensibly it was to sell bonds
on Wall Street and escape the known and predictable land of the
midwest.

In a larger sense, it was obviously so that the
story could happen.  The midwest was, as I said, predictable, honest, not intriguing. 
The creation of Jay Gatsby needed the midwest as a place where the idea was born, but he
couldn't reach out to the world and attempt to squeeze into the world he wanted to be a
part of in the midwest.  So Nick had to move out east so that he could (and we by proxy
could) meet Gatsby and tell the story.

It isn't all that
different from our impression of the Midwest today.  It is a place where things are
bound by tradition, values, things move more slowly, etc.  Though many of these are
simply perceptions and impressions, again, the story couldn't have happened there in
Fitzgerald's mind.