Hi there, while you wait for more answers you may be interested in these previous, similar threads:

https://www.reddit.com/r/AskHistorians/comments/3nc9nb/why_did_world_fairs_stop_being_a_big_things/

https://www.reddit.com/r/AskHistorians/comments/1ie5u4/what_happened_to_the_worlds_fair_what_caused_its/

https://www.reddit.com/r/AskHistorians/comments/4nyuhm/when_did_the_worlds_fair_stop_being_a_big_thing/