Making great films is rare. Even good ones. 9 out of 10 films are lousy to okay, and the 1 odd films out is good. About 10% of that 1 out of 10 is great.
Star Wars was the most popular and highest grossing film ever made, and one that has such power that it has shaped the very fabric of the culture that embraced it; amazingly, it is also unanimously hailed by critics as a masterpiece, something that is all the more rarer. Lucas captured lightning in a bottle with ESB, a film many fans and critics feel is in many ways superior to even the first film. The only other film series to do this really is Godfather, and maybe Lord of the Rings. Because of this incredible--and indeed, seemingly impossible and unprecedented--track record and the fact that these two films were now part of a larger narrative, the bar was raised for the subsequent films and those same films were faced with the even greater challenge of linking up and completing the narrative which was began in 1977.
Very unsurprisingly, the final installment of that initial trilogy did not meet the standards of the first two. Normally this would be though of as "duh, what else did you expect," but the fact that ESB met--and perhaps surpassed in some peoples opinions--the original made us believe that the rest could keep up this standard. If ESB had been "average" the way ROTJ we would have gone "meh," enjoyed the original for the masterpiece it was, and painfully acknowledged that a bunch of sequels were unsurprisingly made for it that werent very impressive but had their moments. Like Jaws. Like the Exorcist. Like Rocky. Like Superman. Like The Matrix.
But people enjoy ROTJ because it survived on the coattails of ANH and ESB. It was just average as a whole, good in parts and great in moments. But it was part 2 of 2, the resolution of ESB, and in many other ways part 3 of 3, the resolution to the trilogy of which the other parts were two of the best films ever made. So it gets by, even though it is criticised.
The PT is similar to ROTJ, only a bit less dramaticly compelling in its constructed and execution. The OOT doesn't need it, the OOT survives on its own, thus the immunity granted to large parts of ROTJ isn't given. Thus the PT is what ROTJ without any OOT connection, only even more poorly executed--movies that range from lousy to average as whole, with a bunch of really good parts and moments. But every FX extravaganza has its moments. Bad Boys 2 is a piece of shit but i enjoy watching some of the car chases for instance, and the Matrix sequels follow on an almost identical manner.
Really, is it any surprise that in a series of 6 films, two are great, one is okay and three are below average? I think it is incredible that not just one is great but that two are. I'm sorry but its just so impossible to consistently make films as good as the first two--we got spoiled by ESB because if it wasn't for that then we wouldnt get so attached to "the saga." What makes the PT particularly tragic for its maker, who truely tried his best, is that while ROTJ is accepted into the great pantheon of ANH and ESB to form the original trilogy, the OT doesn't depend on the PT at all, and thus the films are exposed for what they truely are and tossed aside (after spending millions of dollars at the box office and home video of course). But people go to great pains to like the PT and include them simply because the OT is so great, because that first film was so powerful and the story presented in the one great sequel and one okay sequel so fascinating that we can delude ourselves to accepting a quality that we would ordinarily dismiss simply because we love the world and characters introduced way back in 1977. Thats really all there is to it.
Also, some people just have shitty taste. I mean Bad Boys 2 and Lucky Number Slevin do more business than any film by Wong Kar Wai and PT Anderson. Most people are just fucking dumb when it comes to entertainment, and the amount of people who would claim such average films as the PT as "great" is consistent with the idiocy proved in numbers at the box office.