Ah, my area of expertise!

I looked through the earlier threads (linked by lamapizza) and read the comments that say the Worlds Fairs (Expos) never really ended and continue to draw large throngs... ... and while this is not exactly wrong, I don't think it's really right, either.

Raw attendance numbers are misleading. For two reasons: first, the population has grown dramatically, transportation is cheaper, faster, and wider-ranging (you can fly to Europe or China in a day), and more people today have a great deal more disposable income. So it's much, much easier for people to attend an Expo today--and, correspondingly, this 'ease' means it takes less of a financial investment and personal 'commitment' to attend.

But secondly the cultural impact of the present-day Expo is fairly minimal, at least relative to the past. They are no longer the centerpiece of the news (where is the most recent Expo? I have no idea.) More importantly, they are no longer the core of both entertainment and edification that they once were.

As someone who's researched worlds fairs/exhibitions a great deal, I would say that their peak, in terms of culture stature and societal "impact", was the 1880s and 1890s.

The smashing success of the Crystal Palace in 1851--which was The Big Thing in Europe--sparked imitation and the gradual escalation of Worlds Fairs, of course. Particularly prominent ones were Amsterdam 1883 (which brought a whole exotic/colonial theme into play), Paris in 1889 (the Eiffel Tower), and Chicago 1893 (Columbian Expo--of "White City" fame).

But really, by the First World War, you already begin to see a waning of interest. Again, not represented by raw numbers--cheaper rail tickets meant more and more people could attend. But rather, in terms of cultural impact--the way that they were no longer the centerpiece of cultural 'happening' anymore. They weren't talked about, written about, or obsessed about in the same way in 1910 that the had been in 1885. By the 1930s, great exhibitions were definitely on the wane.

I would argue that this has everything to do with the rise of the inexpensive illustrated media. (lithography, chromolithography and offset lithographic printing)

Here's one example of why: the International Colonial and Export Exhibition really put Amsterdam on the international exhibition map in 1883; it included several reconstructed "native villages", where people from Africa and Southeast Asia were brought to Europe, set into 'authentically' reconstructed villages, and "performed" their culture for assembled throngs. These native villages became hugely popular over the next decade: if you cannot afford to visit Siam, then why not bring the Siamese to you??? A taste of the exotic. And for the vast majority of people, this was the only way they would ever see these exotic places.

But by the 1890s, you could buy an illustrated magazine that showed pictures of these places. (National Geographic first appeared in 1888) At first, these were engravings, but after 1900 or so, they were increasingly photographs (reproduced at first by copy-engraving, then directly in offset printing). By the 1930s, photos had become the mainstay of magazines.

So why go to the hassle and expense of going to see a staging of a "reconstructed" native village in a city, hours away,... when you could now see photographs of the real thing from the comfort of your own home??

Same with the other bits of exhibitions' bread-and-butter...
For instance, the new technology magazines like Popular Science (founded in the 1870s, but taking off in the 1890s with cheap illustration) you no longer had to go to the exhibition to see the latest wonders of science (like telescopes or x-ray machines)... you could see them in print far more easily, delivered to your home.

Then, of course, you have film (movies) taking off after the first World War, with thousands of movie-houses in the US and Europe by the 1920s. This was the death-knell of exhibitions.

In fact, the relationship between printed illustrated media--in the way it supplanted the exhibition--is a bit like the way film took over from the stage;
you still get many people attending plays today, of course, and in large numbers... but movies had definitely eclipsed plays by the 1950s.

For exhibitions, see Robert Rydell All the World's a Fair and (implicitly showing my point about media replacing fairs) Thomas Richards The Commodity Culture of Victorian England

Have a look at this thread where the same question is asked with some pretty good answers from /u/TheEmperorsNewHose and /u/FalterFire