ESB is unique in its production for so many reasons. Lucas' initial conception of the sequels was that the series was to be a bit like James Bond, that each episode would follow different characters, different eras (including the "young Obi Wan" days, as he described it in 1977) and have different tones. Every few years a new director would have a go in the galaxy, putting his own stamp on the film, and Lucas would essentially step back after setting up the basic story. In this, Kershner was pretty much the only person who was allowed to play in Lucas' world, given free creative reign and little opposition; Lucas only visited the set three times. And look how it turned out! But after Lucas hired Kershner, things began to change--first, in the story itself. When the story conferences started in late 1977 it was to be like a James Bond series, but by the final draft it had changed to a 9-part dramatic narrative where Darth Vader was Luke's father (this particular change occured in April of 1978). And then the fiasco of ESB went into production--the film went massively behind schedule and over budget (it DOUBLED!) and the stuff Lucas saw in dailies was completely opposite to his style, and Kershner and Kurtz refused to bow to Lucas' will.
Then the film was released--it did okay, pretty darn good actually, hitting just over the $200 million mark. But it had some criticism and its gross was a whole $100 million less than the first film and kids were not as enamored with the content.
Then Raiders happens. Raiders was essentially the replacement Star Wars sequel--while ESB had progressed to a more epic, Dune-like "saga", Raiders picked up where Star Wars left off, a brainless, swashbuckling saturday matinee serial, devoid of anything particularly profound and told at a breakneck pace with constant action and flash. And without any built-in audience at all it made a whole $50 million MORE than ESB.
Do the math.
The Star Wars sequels primary purpose was to fund the $20 million Skywalker Ranch, as he admitted multiple times back then--although Lucas obviously cared about the quality of the content of the films, it was only to a certain point. If kids were getting bored and profits were dropping then a different route had to be taken. He hired a puppet director (i have a quote from the 1983 issue of American Cinematographer where Kazanjian says something to the effect of "George wanted a younger director that wasnt so well established and wouldn't be as imposing of his own ideas on the film" lol) whose style was closer to his own; Lucas was on set every single day, directed actors and second unit and was for all-intents and purposes, a co-director. And it shows--ROTJ feels very much like a Lucas-film.
Want to reach the kiddies? ESB had only one grumpy, Buddhist-philospher puppet--now he gave them a whole palace of them! Plus a whole planet of teddy bears! No wonder kids loved this film--and its box office gross reflected it, surpassing both ESB and Raiders. Hurray for $$! Too bad Marcia Lucas took most of it a month later!