Lucas knew the kind of story people wanted.
A few days ago I re-watched the movie Fox thought was going to be their sci fi blockbuster of 1977. Damnation Alley is a strange badly made pile of giant scorpion droppings with no redeeming value and up until the bizarre tacked on ending, no hope. The first Star wars movie is called A New Hope. you expect up's and downs, of course. Everyone enjoys a tragedy on some level. But there has to be something gained from it. There was a lot gained from Empire Strikes Back. What was gained from Revenge of the Sith? It wasn't as bad as Attack of the Clowns. But even it is much better as it at least fleshes out an interesting time period, a kind of corrupt civilization that was going to be swept away one way or another, either from within or from without.
Last Jedi is just having your metaphorical head held in a toilet while Jonson drones on about iconoclasm.
IMHO Episode VI was a good, but not great movie. It's fundamentally just a westernized samurai movie, with the "monomyth" as the main plot arc, with really, really excellent design work done. It was the right movie at the right time however, which is why it took off like wildfire.
The Empire Strikes Back was a fantastic movie, however - in large part because Lucas didn't write the screenplay.