The fact that all five SW sequels/prequels were mega blockbusters but still didn't sell nearly as many tickets as the first film shows that the first Star Wars was a mega-super-dooper-phenomenon to the 15th power! It was a phenomenon beyond description. Also, I think one of the reasons is so many more tickets was that it was the first of its kind and so almost everyone and their grandmother saw it at least once. It did tons of repeat business, stayed in theatres forever and just kept selling ticket after ticket. When Empire came out, you'd have to figure that maybe 20% of the people who saw the first film didn't like SW enough to want to see the sequel and of the people who did see Empire, most people probably saw it once or twice instead of 10, 15, 20 or 30 times.
And in terms of the Special editions, the films came out every three weeks, so it was a lot to ask of people to shell out money for all three SW films in a period of 6-7 weeks when most people owned the movies at home. Regardless, the Empire SE still did much better business than the ET rerelease in 2002.