Lucas at least had a plan and story arch to the OT and Prequels. It wasn’t always executed the best and had its ups and downs but the story was there usually with plenty of lore to back it up.
This is not entirely true and subject to a LOT of revisionist history by Lucas himself.
His original treatment was a convoluted mess, until some of his filmmaker friends (Francis Ford Coppola, for one) talked some sense to him. Then he abandoned his original story outline and created a self-contained, one-off story for Star Wars on the assumption that this was his one chance to tell the story.
When it became clear he was going to be able to make more films, THAT'S when things got messy. Vader was never intended to be Luke's father in Star Wars (now hastily renamed to A New Hope). In fact, the original screenplay for Empire featured a conversation with Luke and his Force Ghost dad, Anakin, who was decidedly NOT Vader. And the mention of "there is another" in Empire was never intended to be Leia -- it was going to be a completely new character. It was around this time that Lucas started giving interviews stating that he planned a total of 9 films, 3 prequels and 3 sequels. This "other" was supposed to be the focus of the 3 sequel films. while the prequels were going to be the adventures of Anakin and Obi-Wan.
When making Jedi, Lucas realized that introducing a new vital character in the third film would be messy. He was also giving up on his notion of making 9 films, so he revised his plan again and made Leia the "other" and suddenly Luke's sister. But even that plot thread was reduced to the point where it became nearly inconsequential to the overall story. It was meant to be a revelation on the level of Vader being Luke's father, but at the time of the release I remember it falling kind of flat.
By the time the 90's rolled around, Lucas had entirely abandoned his notion of 9 films to the point of denying that was ever the plan (despite many interviews contradicting this). Now insisting that his plan all along was to tell the story of the rise and fall of Anakin.
The point being, the original films were full of nearly as much WTF pulling the rug out from under the audience moments as the new trilogy, but audiences have been much more forgiving of it over the years. The original films are full of plot holes, inconsistencies, and just really weird choices, but we love them anyway.