I think the sequels to Star Wars (1977) were both a response to analysis of it's success.

ESB is pretty much a condensation of what made the story work and ROTJ is a condensation of what made the film financially successful.

One is a plot and character driven, the other is driven by action and resolution.

As a closed unit the first film has all these things.

I suspect that one of the reasons why in an age where films can be re-watched with ease, ESB is usually more of a hit because plot and character nuances take more time to digest, where as a simple roller coaster can be habituated by a viewer more quickly.