Maybe the title of this thread should read, "When did Empire become held in higher regard than Star Wars on the internet?"
Michael Kaminski aka zombie did a great two part article on the reception of the original trilogy 1977-83 which goes into depth of it: http://secrethistoryofstarwars.com/originaltrilogyreception1.html
Personally, I think many factors play a role other than already mentioned, in the last 15 years, Empire is also seen as the least butchered film in its SE form, it may also affect peoples view but I must say that IMO it was equally fucked up by Luke's scream and destroyed editing.
Another theory of mine is the constant growing anger and hate fans have with Lucas of today, where they often attribute everything that is good about SW or Indiana Jones for that matter to everyone except Lucas, nothing strange really, I hate what a fucking asshole Lucas is today but I will never forget that he is the genius behind THX 1138, American Graffiti, Star Wars, Empire and Raiders, what this man is doing today have nothing to do with his talent of yesterday IMO. The three K's, Irvin Kershner, Lawrence Kasdan, Gary Kurtz were largely responsible for the quality of Empire, but so was Lucas no matter what people think of this man today. But the fact that this was the film he was the least involved with when they shot it, makes this also the most pure to those who think that "the talentless Lucas couldn't ruin this one". It's sad, but this view exist.
The most common view I usually hear is that Empire is the greatest but Star Wars is their favorite, I share that same view, it always was a tie between these two for me growing up but I must admit that I think Empire is the better film now. But Star Wars is still my favorite.
I must also agree with georgec, that the last 40 min or so of Empire are cinematic perfection. Music, editing, performance, cinematography, and special effects are in perfect alignment, I actually believe I have just seen the greatest motion picture ever, every time I see Leia & Luke looking out in the vast space as John Williams beautiful score play. The build up to that cliffhanger is a master stroke, almost too good. Williams is of course a big part of that.
I will however never like that muppet movie where Luke Skywalker has the same hairdo as aunt Beru and where he and Leia are siblings. ;) But in the end, who cares what other people think, as long as you enjoy the movie, others perceptions of it is worthless.
Now, if we all could just enjoy all these films at home in good quality, SE or original, Lucas has made that thing very difficult with all his films.