The reason being is that "Empire" is the middle movie of a series where most of the action takes place. The first movie, like the beginning of most trilogies sets the stage and largely shows that the idea has potential and some big ideas at work. "Empire" sets everything up, creates the jeopardy, and ends at a cliff hanger. The third movie, "Return Of The Jedi" which is perhaps the weakest one is the movie that has to outdo the others, beign the final act, where everything needs to be brought to a final and triumphant conclusion that was worthy of both the initial premise and promise, and the build up in the second movie. Like many things, Star Wars flubbed the ending, but didn't do so on a spectacular enough level to ruin the rest of the series, unlike say Mass Effect. It ended on a high enough note where you could still care about what happend in the other movies.
To some extent the "best" movie in the series was as the middle chapter the easiest one to make, because they didn't need to worry about the set up, or concern themselves too much with making a workable finale. Star Wars being pretty much forumula for trilogies or series.
The biggest problem with Jedi I think was specifically that they couldn't ever make things as cool or dramatic as the second movie. In the final equasion the stupid ewok puppets could not produce as awesome a battle as what we saw in "Empire", and the final scene between Vader, The Emperor, and Luke which should have been the most awesome confrontation in the entire series, making the battle with Vader in Bespin look pathetic in comparison, was kind of cheezy and fairly anti-climactic. Sure it was dramatic enough in it's own way I guess, but didn't quite achieve highs equal to or surpassing the lows brought about by the second act.
That's my take on it at any rate you had a good movie, an excellent movie, and a fair movie.
You'll also notice that this is par for the course with most things, to the point where the exceptions really stand out. Take "Lost" for example, the set up for the series captured imaginations, the middle parts of the series were the easiest to make because all they had to do was keep shooting out wierd crap left and right, and worry about tying it all up later once their audience started to get bored. When it came time to end it and tie all of that together they failed to create an ending that was worthy of all the stuff they had going on in the second act (so to speak) and thus it ended on a sort of "meh" note.
Star Wars did have the advantage of doing so well at it's good moments that people wanted more of it, long after it was done (as we all know) which is rare. The epic moments it delivered, in all three movies, can make it easy to forgive it's downside. In the ending the final scene of Vader chucking the emperor into the reactor core for example was cool enough even in the height of highly contrived cheese that it was enough to make you want to forgive how utterly tortured that entire scene/confrontation was until it got to the point. All meaning aside, some goober in a robe going "embrace your hate" just wasn't going to match the awesome inherant in the series other confrontations, even with the quick battle with Vader that took place and the arm-cutting parallel.
Such are my thoughts.