If you weren't alive in 1973-74, you probably have no idea...
...how big this movie was that year, or how "blockbusters" in general were treated in the days before multiplex theaters, VCR/DVRs, and the Internet. Most theaters were still single screen, one movie at a time operations then, and prior to VCRs in the '80s if you didn't see a new or recent release while it was in a theater you didn't see it at all until it was shown on TV years later, if ever. All of those factors led to a much greater demand to see movies while they were in theaters, particularly those that got the kind of positive press and word of mouth received by The Sting.
The demand for this movie among people who not only saw it once but over and over again was so great the theater near where I lived showed it exclusively and continually for over a year, and it was not the only theater to do so. Think about that - first-run, single screen movie theaters showing one movie only for a whole year. That tells you the kind of ticket sales this movie generated over a sustained period of time, and how many people came back to see it over and over again. Even among the so-called "blockbuster" movies, there were few that generated the kind of sustained, multiple viewing tickets sales produced by The Sting. Like many people, I went back to see it at least three times that year and each time picked up details in the plot that I had previously missed. I still watch at least part of it any time I see it on TV. Great movie.
vvv WARNING - THAR BE SPOILERS AHEAD vvv
Another indication of how this movie was received was the fact my 9th grade English/drama teacher took the whole class on a field trip to see this movie soon after it was released. In addition to getting out of school to go to a theater in the middle of the day, I enjoyed the movie itself (particularly the stripper scene), but some of the plot twists went over my 14 year old head that first time I saw it. I remember the gasp that went through the audience when "dead" Redford opened his eyes at end and those in the crowd who had never seen it before realized they had been conned along with the bad guys, and the "Wait a minute! You mean so-and-so was in on it all along?" conversations on the way out of the theater.