Why do I love Star Wars? The same reasons everyone else does: incredible imagination, an interesting plot, great action scenes and special effects and loveable characters. Basically, thats why the film(s) are successful. You could probably analyse it as a four-stage design of success:
Firstly, because its exciting. Chases, explosions, shootouts, swordfights, absolutely fantastic special effects--these are the reasons why we are all initially captivated by the film. Its designed in a very calculated graphical sense to be visually stimulating. The film is filled with imagination and action, with aliens and creatures and spaceships that travel at the speed of light and doomsday weapons that blow up entire planets, and its presented at a very quick pace--the audience always has their attention stimulated.
Secondly, the storyline--a fantastic tale that is ingenious for its use of archetypal themes and plot points and updated science-fiction/fantasy interpretations; you got everything from knights of the round table to John Ford's old west to Buster Crabbe and the Flash Gordon serials, and they are all weaved together in a seamless blend of archaic and modern storytelling. All these dressings however still would not be effective if the true through-line of the film, the rise of Luke Skywalker, was not compelling. The back-story and environment are complicatedly designed so as to be rich and full but the heart of the film is quite simple and easy to follow.
Which brings us to the third point, the characters. This is the true hook of the films. All the fancy special effects, stimulating explosions and intricate plotting would be for naught if the characters weren't compelling, and this is where the original trilogy leaves behind the prequels. The characters were warm and funny, fully rounded and three-dimensional, written with wit and intelligence and acted with realism and sincerity. They were relatable and believable. This above all else is why the film was successful. They are the heart of the story, and in spite of the over-the-top design of the film, the characters are so sharply defined that are able to penetrate through the overwhelming material and become the focus of the film.
Fourthly, the spiritual themes and optimistic message made you feel good. At its most basic, Star Wars was about believing in yourself, about the power of positive thinking--Luke uses the Force because he believes he can, he defeats Darth Vader because he believes in himself. Star Wars, at the end of the day, was the ultimate feel good movie, the story of the triumph of good over evil. This is what kept people coming back when other great character-driven action films faded away, to delve into the mythology, and to be re-affirmed in their belief in the world.
All these things combined made the film a huge hit with audiences in the historical context in which it was released, but these things remain to this day as the basic formula for an audience pleaser. A clever and universal story, well told and well crafted, with terrific characters and optimistic spiritual themes, filled with action and excitement and imagination and with eye-popping visual effects. What more could you possibly want?