Darth Vader is one of the mightiest users of the Dark Side to ever exist. Therefore, it might seem strange that not once since embracing the darkness did he conjure Force lightning -- one of the trademarks powers of the Dark Side -- in any of the Star Wars films. Force lightning. While Palpatine is most famous for employing it, various Dark Side users such as Count Dooku, Mother Talzin, Snoke, the Son and even Rey use Force lighting. So why not Darth Vader?
A user on Quora asked this very question, leading to fans digging throughout modern and Star Wars lore to find an answer. The results of his investigation led to a straight forward conclusion: Vader couldn't use Force lightning because it would kill him.
IT WAS VADER'S CHOICE
The first matter of business is that Vader is able to use Force lightning, but he simply chooses not to. Force lightning comes in many varieties, with the most powerful being the red lighting generated by the Son in The Clone Wars series, implied to be a manifestation of the Dark Side itself. Even Palpatine's lightning, which can subdue an entire fleet of ships in The Rise of Skywalker, pales in comparison to the Son's potential power. No other character uses Red Force Lightning ever in the series other than the Son ... except for Vader.
In Marvel's Darth Vader: Dark Lord of the Sith #25, Vader sends an astral projection of himself through a portal in his castle at Mustafar. He encounters an illusion of Palpatine behind this portal, preventing him from seeing Padme. Vader responds by annihilating his master with Red Force Lighting, proving that not only can he harness the power but utilize a version of it mightier than what Palpatine wields. With that in mind, it makes one wonder why Vader didn't use the ability during the Original Trilogy.
MORE MACHINE THAN MAN
Vader's Force powers exceed beyond Palpatine's in this regard, but the reason why Vader can only channel force lightning in his astral form is that using it with his physical body could prove fatal. As seen in Return of the Jedi, even a little Force lightning is enough to overload Vader's circuitry and life support systems and put an end to his life. When using Force Lightning, Vader allows the lightning to flow throughout his body. That sort of energy channeled throughout his entire system, would prove fatal to his circulatory system.
However, there's also the possibility that Vader in his physical form just can't generate Force Lightning at all. In Star Wars: Revenge of the Sith: The Visual Dictionary, it is stated that Vader's robotic limbs cannot channel Force energy. Force lighting is often channeled through a person's hands, and Vader lacks those. With his robotic limbs, the force can't flow through him at all, leaving him incapable of channeling force lightning in his physical form. However, this explanation may no longer be considered canon after the Disney relaunch.
KINETITE
While lightning flowing through Vader's system would prove fatal, that doesn't mean that Vader can't use Force lightning at all. Rather than channel it throughout his person, he generates energy outside of his body in the form of Kinetite. This power is a sphere of pure kinetic force that only resembles Force lightning superficially.
While briefly touched upon in the Disney canon (more specifically, Star Wars: The Rise of Skywalker: The Visual Dictionary), Kinetite is present throughout the Legends lore. Vader empl-oys Kinetite first in the comic adaptation of Splinter of the Mind's Eye before using it again in Darth Vader and the Ghost Prison and Star Wars: Masters of Teras Kasi.
There are conflicting explanations as to why Vader can and can't do this. In Star Wars Insider Magazine #80, it is implied that the field isn't created through the Force but rather through devices on Vader's suit. The Rise and Fall of Darth Vader rationalizes Vader's use of Kinetite by claiming it's his channeling the power of Splinter of the Mind's Eye's Kaiburr crystal (not to be confused with the kyber crystals) with his Force. Ultimately, the only way Vader can employ Force Lightning is by using remotely channeling the energy outside of his body.