Whatever Became of The World's Fair?

This amazingly detailed diorama of New York and its five boroughs was created by Robert Moses for the 1964 World's Fair, which got me thinking about why the World's Fair ceased to exist. After a little research I discovered that they never did go away, but have only left the term "World's Fair" in the dust for the more modern and techy title "Expo". Unfortunately, this title is about a tenth as fantastical as its predecessor.