> what happened to world fairs, and why don't we have more of these?

We do, just not every year. [0]

Expo 2000 in Hannover, Germany, was a very big deal. The tram/metro/bus system of this 500k pop city was significantly improved. Infrastructure was modernized.

At the exhibition you could visit very creative pavilions that tried to capture something like a national spirit (or whatever the national committee wanted to represent as such). Switzerland and Japan were my favourites. [1] The former fair grounds and some structures built specifically for Expo 2000 are still being used today for events [2].

  [0]: https://en.wikipedia.org/wiki/World%27s_fair#World_Expos_2
  [1]: https://en.wikipedia.org/wiki/Expo_2000#Pavilions and https://en.wikipedia.org/wiki/Expo_2000#Gallery
  [2]: https://expowal-hannover.de/