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Answer by Rob Fletcher, Star Wars geek:
Because in Rogue One he benefited from 21st-century special effects and the weight of expectation brought by almost 40 years of being the most iconic villain in movie history, while in Star Wars he was in a cheap sci-fi flick nobody expected to be a hit and trying not to break the flimsy lightsaber prop.
We watched Vader grow in power and mythic status through the original trilogy. By the end of it he was stamped indelibly on movie history. He was so unforgettable that 16 years later an entire trilogy could be justified to tell his back story. His power was undimmed despite that prequel trilogy depicting him as a gee-whiz kid then a leering, whiny teen, fumbling his turn to evil then only showing him in the full costume at the very end in a scene that was unintentionally funny.
Another decade later and a new generation of Star Wars movies decided (rightly) that they couldn't out-Vader Vader, so the new masked villain is one whose greatest weakness is his own inadequacy in the face of Vader’s legacy.
Then along comes Rogue One. Because of when it's set and how it ties to the original movie, it's logical Vader could, even should, appear. But how can the movie use Vader? He has to have impact, but he can't overshadow the A cast. Any fanboy will tell you the thing we'd never seen was Vader just straight wrecking some dudes. We'd seen him fight with a lightsaber on one-on-one duels but only against trained Jedi. The scene they came up with certainly has impact. It's horrifying, crowd-pleasing, and perfectly brief.
Forty years ago Vader wasn't Vader yet. He wasn't Luke's father, and he wasn't in the middle of a tragic fall-and-redemption arc spread across six movies. When they filmed Star Wars the lightsaber duel was a kendo-inspired sword fight. Jedi hadn't yet developed the acrobatic fighting style of the prequels or even the heavier, medieval sword fighting style of Empire and Jedi. The spinning wooden rods coated in reflective tape were prone to shattering on contact, so the actors had to be careful. Forty years later Vader’s cultural import is enormous. Audiences would feel cheated if his return to the screen wasn't given a spectacular, grandstanding scene commensurate with his status.
Why was Darth Vader so powerful when he confronted the rebels in Rogue One compared to his confrontation with Obi Wan in A New Hope? originally appeared on Quora. More questions on Quora: