World’s Fairs are definitely still held. Though often called world's fair in English, the official titles of many events were "Exposition Universelle" almost from the beginning, and that's the designation eventually adopted by the sanctioning organization, Bureau International des Expositions, in 1928. Of course, the 1893 fair in Chicago was officially the World's Columbian Exposition, and Expo '67, Expo 1970, etc., continued this naming tradition. Cities or nations decide it will help to promote their locations and organize a Worlds Fair, and (since 1928) apply for recognition as a registered exposition.

As for the decline in attention, the trade show aspect (incredible new steam engines! super-sized canteloupes!) was supplanted by industry-specific expositions. The aspect of displaying unusual foreign cultures and faraway lands came to seem exploitative, and didn’t interest nations who wanted to demonstrate how modern they were, not how colorful. In addition, it became possible for large numbers of people to simply visit other countries instead of viewing a reconstructed village. The midways full of thrill rides were supplanted by theme parks such as Disney and Six Flags. Television reduced the curiosity-seeking value of static displays, and pavilions began to simply be theaters for viewing television programs. Late-20th century expos, especially in North America, seemed overwhelmed by the corporate pavilions, which were of dubious marketing value to the companies and seemed a little distasteful to attendees.

Morris Dickstein expands on these theme in his essay “From the Thirties to the Sixties: The World’s Fair in its own Time,” published in in Remembering the Future: The New York World’s Fair from 1939 to 1964.

Previous discussion from 2013.

[Previous discussion from 2015.] (https://www.reddit.com/r/AskHistorians/comments/3nc9nb/why_did_world_fairs_stop_being_a_big_things/)