Why Didn't Darth Vader Upgrade His Suit?

Vader kept the suit because there was no good way out of it. In Legends he longed to be less dependent on it and tried at times to manage without it, but never succeeded, and the flawed linkages kept him tied to Imperial medics for the rest of his life. The armour also kept him in constant pain, and that pain was what kept his rage, and his power in the dark side, burning. (Star Wars: Darth Vader: A 3-D Reconstruction Log; Star Wars: Shadows of the Empire)

The suit was never built for combat#

What Darth Vader wore was a life support system with armour bolted to it. It carried prosthetic arms and legs, a full breathing apparatus, and enough hardware to bring his mass in armour to 120 kilograms. Everything about it was heavy and deliberate. The man inside had once been one of the fastest duelists in the Jedi Order, and the suit took that away from him, forcing him to change his lightsaber style to cope with its weight and stiffness. (Revenge of the Sith; Star Wars Blueprints: The Ultimate Collection; Dark Lord: The Rise of Darth Vader)

Legends sources are blunt about how exposed it left him. Palpatine knew the armour was vulnerable to electrical discharge and used the threat of Force lightning to keep his apprentice in line. The Jedi survivor Roan Shryne worked out during a duel that the chest-mounted control panel was a weak point, and that Vader's whole fighting style prioritised protecting his centre over protecting his limbs. That is not a warrior choosing an aggressive guard. That is a man defending the machine keeping him alive. (Dark Lord: The Rise of Darth Vader)

Nights were no better than days. The armour did not repair the damage done on Mustafar, it just held the line. In Legends, Vader experimented with dark side healing during his meditations and could make his ruined lungs work unaided, but only for minutes at best. The relief itself was the problem: the moment of happiness broke the concentration on darkness that the healing depended on. He believed that with enough willpower he would one day manage it permanently, and he never did. (Star Wars: Shadows of the Empire)

The way out he never found#

The obvious fix was a better body. Nothing in Legends shows Vader getting one. The suit was damaged and repaired, and upgraded in places, several times over the years, and Vader added some insulation once he learned it was vulnerable to electrical discharge, but its core design stayed, flaws and all. (Star Wars: Darth Vader: A 3-D Reconstruction Log)

Any rebuild would also have meant more surgery, and Vader had already been through one reconstruction that left him hollowed out, carried out without anaesthetic. Putting what was left of his body back on a surgical table meant handing his fate to someone else, which is the last thing a Sith Lord in Palpatine's court would ever do willingly. (Star Wars: Darth Vader: A 3-D Reconstruction Log)

Pain was the point#

The second reason is the one people miss. The suit hurt, and Vader needed it to hurt. (Star Wars: Darth Vader: A 3-D Reconstruction Log)

The dark side runs on emotion. Anger, hatred, fear and pain are the fuel, and Vader had a permanent supply strapped to his chest. Every breath was a reminder of the duel that ruined him and of everything he had done since. Take the agony away and you take away the engine. In Legends that was a deliberate choice from the start: no anaesthetic was used during his reconstruction, on the Emperor's orders, to deepen his connection to the dark side. (Star Wars: Darth Vader: A 3-D Reconstruction Log)

There is a quieter cost too. Less physical pain would have meant a clearer mind, and a clear mind had nowhere to hide from memory. The man who could think straight was the man who would start thinking about Padme Amidala, about Obi-Wan Kenobi, about the Jedi he used to be. Upgrading the suit would have made Vader a stronger duelist and a weaker Sith, because it risked bringing Anakin Skywalker back to the surface. Vader was not interested in meeting him again. That last part is our reading, but it fits how fragile his dark side healing proved. (Star Wars: Shadows of the Empire)

Palpatine wanted him limited#

There is also the matter of who signed off on the armour in the first place. Palpatine had the suit built, and the medical droid who oversaw the work suspected its faulty linkages were left that way on purpose, to keep an apprentice dependent rather than create a rival. (Revenge of the Sith; Star Wars: Darth Vader: A 3-D Reconstruction Log)

The contrast with General Grievous makes the point. In Legends, Grievous was rebuilt on Geonosis into a duranium alloy body with arms that split at the shoulder into four, enhanced equilibrium to make him faster on his feet, and organs sealed in a synthskin sack that could survive vacuum. That was a machine designed to hunt Jedi and win. Vader's suit was designed to keep a man breathing, and Legends even has the same droids that rebuilt Grievous do the work. (Star Wars: Revenge of the Sith: The Visual Dictionary; The New Essential Guide to Droids)

It's tempting to say Palpatine learned from the Grievous experiment. Dooku had built a superior cyborg and ended up with a commander who resented his handlers, and nothing suggests the Emperor wanted that from his own apprentice. Under the Rule of Two, the apprentice is always waiting for the master to slip. Vader himself told Sidious he would not kill him until he had learned everything he knew, and Sidious kept the threat of Force lightning, and the suit's weakness to it, in reserve. (Dark Lord: The Rise of Darth Vader; Unknown Soldier: The Story of General Grievous)

What it cost him#

The result is one of the strangest trade-offs in the saga. Vader spent more than two decades as the most feared being in the Galactic Empire while operating at a fraction of what he could have been. He knew it. Palpatine knew it. Both of them left it alone. (Star Wars: Darth Vader: A 3-D Reconstruction Log; Return of the Jedi)

The suit stayed because it was doing three jobs at once. It kept him alive, it kept him angry, and it kept him underneath Palpatine. Upgrading it would have fixed the first job and broken the other two, and neither Sith Lord showed any interest in that. (Star Wars: Darth Vader: A 3-D Reconstruction Log; Dark Lord: The Rise of Darth Vader)

It only came off once, on the deck of the second Death Star, when Luke Skywalker removed the helmet from a dying man who had finally chosen to stop being Darth Vader. By then the suit had done everything it was built to do. (Return of the Jedi)