In Universe explaination:
In his mechanical life-support suit, it was extremely difficult to engage in the energetic acrobatics of Ataru, an act that would quickly tire him, defeating the purpose of the acrobatic style. For the remainder of his life, Vader maintained his mastery of Form V, but introduced more elements of Soresu and Ataru swordplay into his customized style, as witnessed by Jedi Master Roan Shryne when Bol Chatak dueled with the Dark Lord. It was noted that he would often employ powerful yet sometimes clumsy and stiffly executed vertical strikes to wear an opponent down, which he utilized both against Bol Chatak and Luke Skywalker.
He apparently learned his lesson from his duel with Kenobi, learning how to control his emotions when in combat and finding a way to call upon calculated bursts of the dark side while not being blinded by emotions running amok.
It's just a physical style. Limitations (bulkyness and slow) combined with the suits and Anakin's strenght (physical powers and mass).
I also recall it's based on the real-world 'Kendo' form?