The large sponsorship fee is part of the problem, but there's another complication from the EPCOT Center era. Epcot's World Showcase is essentially a permanent World's Fair, and the Fair guidelines state that any participating country cannot be part of another permanent exhibition. That's why many of the pavilions were originally sponsored by companies, not governments, of each respective country.
That said, World's Fairs are essentially over. The last major one in America was in New Orleans in the early 80s, and it didn't do well. A smaller one followed it almost immediately in Canada and performed even worse. Small expos have been built since then, but they tend to be highly regional. Nobody travels around the world to visit them.
Epcot's WS is practically the last of its kind.
Edit: This year's World Fair Expo is in Shanghai. Nobody even talks about it.