The answer is this: Princess Leia was a character we had grown to care about as fans. With the way she was written as a strong non-damsel in distress politician and how Carrie Fisher portrayed the character perfectly, the fan community grew a connection with the character. And the slave get up was such a departure from what said community knew about her and they liked this dirty side. Basically, the appeal was not a scantily clad woman. The appeal was that of having a best friend you have known for years and grew a connection with. It was not like you did not think about her or her him in that way; you just saw their personality more so than you did how they looked physically. And then comes the time when you go to the beach and see them in something more revealing than you are used to. That feeling of arousal is more so because it is someone familiar and someone you created a mental affection for. That is the Slave Lea appeal in a nutshell. Because, as fans, we do generate some sort of bond with the characters in our favorite series. If we dont, the creators and actors are doing something wrong. When that on-screen character that we had watched grow and develop in a series is presented as a sex symbol, it goes from just fan service to a fan sensation. It goes from just a woman in a bikini to Iconic all because of the time we were allowed to get to know the character for the character and NOT as a fetish or sex symbol before the aforementioned scenes in Jabbas Palace. That is why the Slave Lea costume is such a testament to how great the original Star Wars series was. If the series was not well written, if we did not care about the characters, and if we were not compelled by the story and the universe that it existed in, it would mean nothing. It would mean just as much as the Padme drift scene. Nobody cared about that trilogy; nobody cared about that character. It was just a very attractive young lady with a mid-drift. But with the Return of Jedi get up, we care, therefore it is sexy. Does this mean that the outfit is not a pervy nerd fetish? No, it still is. Does this make it the ultimate nerd fetish? Yes, it most certainly does. It will probably be for years to come, unless Disney somehow tops it with something in Episode 7. But it is hard to compare to that original moment and original costume that made fans realize how infatuated with the character they really were.
