Apologies for this long-ass post:
So I think I figured out one specific thing I can point to which serves as the perfect razor to say why the OT holds water so much longer than the prequels or sequels: simply, every single scene in the OT moves a character forward in some way. Like no joke, each scene we learn something new about a character, or see them change in some way, or reveals new depths to an established trait which we wouldn't expect.
Seriously, play by play the scenes of ANH and what we learn about the characters, starting from when Luke cleans C3PO and R2D2:
Cleaning C3PO and R2D2: We learn Luke has a fascination with flying. We learn he's not satisfied with his life and wants a life of adventure. We learn he's more personable with droids than the society around him, which establishes his kind nature. We learn of his passion for the Rebellion vs. the Empire and his curiosity about the battles. We learn Luke is thirsty. We learn a tad later that R2D2 is capable of deception. We learn C3PO is subservient.
Meal with Owen and Beru: We learn Luke has curiosity about his father. We learn Luke is trying to get out and join his friends. We learn that Luke's father has some kind of past that people don't feel prudent to share.
Searching for R2D3: We learn Luke understands responsibility but is naive. He then gets up early to look for R2D2, showing again his sense of responsibility and duty.
Sand People: We learn Luke is self-confident but also foolish, getting himself into more than he can handle. This reinforces his desire for adventure and excitement, but is contrasted with his naivety.
Obi-Wan enters: We learn that Obi-Wan has wisdom about how to navigate a rough land, and is established as a mysterious dogooder. We learn Obi-Wan has been using a different name, and we are reaffirmed about his wisdom in dealing with sand people.
Obi-Wan's house: We learn Luke has been lied to and believes the lies. We learn of Luke's interest in wars of the past. We learn of Luke's curiosity about his father again. We learn of Obi-Wan's commitment to Leia, and see Luke's apprehension towards leaving. Despite wanting adventure, he hasn't committed yet to the idea of leaving.
Seeing the dead Jawas: We see Obi-Wan's wisdom of the Empire, and Luke naively rushes home. Is finally radicalized towards leaving.
Stopped by stormtroopers: Obi-Wan reveals his power level, Luke has his first experience seeing what he didn't know was possible.
Cantina: We learn Obi-Wan can negotiate, can handle himself in sketchy places, and we see how little Luke understands the underworld. We are introduced to Han, who we learn is a good negotiator. We learn Obi-Wan is self assured of the value of his presence to the Rebellion. We learn that Han is dangerous and indebted.
I can hardly even contain how much characterization and exposition together are completed in these scenes. It's really good. It's just amazing how packed every scene is with expounding information about who these people are, how they interact, how they handle themselves, what motivates them.
Compare that to TFA:
After Finn crash lands: ?? We learn that Finn is a bit upset that Poe just died. We also learn the desert is hot. We learn Finn will drink from gross water if thirsty enough.
Rey fights two dudes: we learn that Rey can fight and Finn is willing to intervene in street fights. We learn that Rey will attack people for having a jacket similar to one her droid says his owner has (?). We learn two things about Finn: that he's willing to intervene in street fights, but is a coward and runs away from street fights (?????????). We learn that Rey beats up thieves (?????????). We learn that Rey kinda cares about the Resistance. We also learn Finn is full of **** and a liar.
Oh wait.. none of this really says much about the characters. All we really learn is that Rey has interest in the Resistance and Luke Skywalker, and that Finn's a liar.
Running from stormtroopers: we learn that Finn wants to do the lead male thing and Rey wants to do the independent woman thing.
Millennium Falcon: Being honest, nothing. It's just a long scene where the characters have no meaningful interaction and don't say or do anything relevant. It's just pointless action.
After escaping: Rey keeps being independent and Finn keeps lying. We learn Rey is impatient and yells under pressure. We learn that Finn is romantically interested in Rey and Rey is autistic.
Hiding on the Millennium Falcon: again nothing really. They just say what they're doing but we learn nothing.
Han appears: We learn Rey has heard legends about Han Solo. I guess that's characterizing, sort of. Among a bunch of pointless nostalgia lines. Han's happy memories of the MF reflect the audience's, so it's not characterizing him so much as it's mildly breaking the fourth wall. We also learn Han Solo doesn't give a **** about some dumb kids.
Rathtars: This scene is stupid ****ing long and, again, we learn nothing about any of the characters. It's just pointless action.
These two list of scenes I picked are about the same into each film and the same overall length. It's like, the sequels do attempt some characterization, but overall it's very little and sparse. Han Solo takes over and when he appears, they just stop altogether trying to build up those characters. It's absolutely horrendous. Most of the scenes are characters just saying things which reflect no internal state or tell us anything about who they are. It's weak, boring writing. If you're like me and waste your time obsessing about these things, try comparing how each scene builds characters and I bet you'll find the better scenes of the sequels are the character building ones, whereas the OT is just character building 100% of the time so it's never bad.