Darth Vader's iconic armor in the Star Wars franchise remains one of the most important aspects of his person. His helmet and life-support armor remain one of the most iconic parts of the character -- alongside his iconic wheezing and red lightsaber. He first donned his outfit following his duel with Obi-Won on Mustafar, where his limbs were severed and his flesh charred. The armor rebuilt his old limbs, restored his healing faculties, and allowed him to take command of one of the Galaxy's greatest fighting forces in history.
However, Vader's armor remains mostly unchanged from film to film. Tony Stark updates his armor multiple times a movie, but Vader appears hesitant to alter a single aspect of his. If you ever asked yourself why Vader wears the same suit decades after the fact, well, there's a reason. At least... there was.
A Few Key Things to Know About Vader's Suit
Darth Vader has one of the coolest suits in cinema history. It's also extremely painful for him to wear it. Vader is regularly in a constant state of pain wearing the thing. The robotic limbs are painful to wear, the suit forcibly nourishes his body, helps oxygenate his blood -- the whole thing is agony. This pain is in part due to how many parts of the armor are wired into Anakin's circulatory system. This is in part because the armor isn't there to look cool. It's there to keep him alive.
Furthermore, the suit cuts him off from the rest of the Force and society. Ever wonder why Vader can't use Force Lightning? He can't channel the energy through his robotic fingers. It also sets him apart socially, making him stand apart from those around him even further than normal. After all, many see him as more of a machine than a man. While he came to accept the isolation he felt in the armor, it still left him to steam in his own resentment toward the world that devastated him.
Still, wouldn't it be possible to transfer his body to a less painful body?
Darth Vader's Armor: The Legends Explanation
In Star Wars Legends, in the novel Star Wars: Death Star, Vader is given a 2.0 suit that would have upgraded much of his abilities. However, the transfer into the suit would have been agonizingly painful. To Vader, there would be little point in doing so, especially considering there remained the possibility he'd not survive the procedure.
On top of that, Palpatine designed the suit to be painful to Vader. After all, the Dark Side of the Force is dependent on anger and hate. The suit represents Vader's crushing defeat at the hands of Obi-Won Kenobi. He wouldn't need a suit to fulfill all his life processes if Obi-Won didn't leave him burning by the lava rivers. This means the suit is, in many respects, an embodiment of his personal failures.
It also doesn't help that the Emperor always planned on replacing Vader. He didn't want to devote much time into an apprentice he planned on disposing of the whole while. Vader served as his top enforcer, but he always wanted someone more powerful -- that is simply the Sith way. With his connection to the Force stunted by his own armor, the Emperor saw Vader as a tool to toss away when inconvenient. This started with Galen Marek -- soon to be known as Starkiller.
Vader's Suit isn't His First
Strictly speaking, the suit Vader wears by the time we see him in A New Hope isn't technically his first. As confirmed in the current canon, Vader's life-support system has been demolished. He's had arms cut off, his helmet blown away, and whole chunks of armor broken through.
In a battle against Galen Marek atop the half-completed Death Star, Vader had his entire body bombarded by a Force-propelled energy field generator. His armor was almost completely decimated in the resulting explosion. This means that everything he wears in A New Hope is different than the outfit he wears at the end of Return of the Jedi.
This naturally leads to the question: why doesn't he update his suit? Why is his suit identical to the one he wore before? Well, in Legends, it is revealed that Vader did update his suit -- albeit mildly, and ultimately, ineffectively. He reinforced his circulatory board in order to protect it against electrical bombardment. He learned that his life support systems would short-circuit should it be overloaded by electrical energy. As seen in Return of the Jedi though, he clearly didn't reinforce it enough, as it is the Emperor's Force Lightning that leads to the end of his life.
Darth Vader's Armor in Modern Canon
Darth Vader's armor has taken far less damage in the modern canon than it has in Legends. For the most part, he hasn't had a single upgrade whatsoever. His current incarnation never even updated his life-support board. The only real damage it ever sustained was in a duel against his former apprentice, Ahsoka Tano. Ahsoka managed to damage Vader's helmet, exposing his human side.
However, much like in the Legends Canon, Vader hated his armor for the first five years of wearing it. After a while, though, he started to actually feel attached to it. He saw it as a way to cut himself off from the outside world in order to hone his concentration on the Dark Side. Vader's power doesn't come from his armor. It comes from his connection to the Force.
This establishes an interesting component to Vader: the armor isn't just a defense against physical attacks, it also protects Anakin from emotional pain. The armor's painful nature gives Vader something very specific to focus on. Vader is able to brood and resent in his own defenses.
However -- and this cannot be stressed enough -- Vader actually has multiple suits he can change into. There are multiple suits of armor Vader wears. Unlike the prior Legends canon, it's far easier for Vader to change his suit. This is in part because much of the life support systems are externalized in the armor rather than built into Anakin's flesh in the new canon. The only exception are needles injected into the base of his spine. These are painful, but allow his brain to directly access his robotic limbs. These are agonizing, but they can be removed.
In the book Thrawn: Alliances, insects on the planet Batuu turn Vader's suit partially into stone. He retreats to his ship, where he dons a new suit of armor. This means that Vader has multiple suits. He just chooses not to change it, because doing so might remove Vader from his private cloister of brooding and meditation. This connection to the Dark Side, and not the physical armor he wears, makes Vader one of the most dangerous threats in the galaxy.