Reading modern history I've often encountered mentions of world's fair events being the catalyst for some important invention or cultural shift.
I've read any cases of this over the years, but some that come to mind right now are:

• claim that NYC's desire to 'clean up' the visible signs of the gay community in preparation to 1964 New York World's Fair may have contributed to Stonewall riots

• Eiffel tower erected for Exposition Universelle (1889)

• 1939 New York World's Fair introduced the concept of television to the public

• McDonald's floating restaurant (McBarge) for Expo 86 in Vancouver

It appears to me that many of these word fairs were cultural moments, comparable with Olympics or (Association) Football World Cup where the collective attention of the world is focused on one place, one event or one thing happening. They almost remind me of how today many people tune into product announcements by companies like Apple or Google.

Yet, I have a feeling that if you asked any Millennial (or younger) what their favourite world's fair was they'd ask you "What's a world's fair?"
Seems that world's fairs have lost all of their cultural cache. Wikipedia confirms as much, with approximately 40 world's fairs being held in 1930s but only 5 being held in 2010s, with a bunch being planned in 90s and 2000s and then getting cancelled.

So, my question is twofold.

a) Is my perception that world's fairs were a huge deal back in the day accurate, and

b) If so, what caused the decline?