Darth Vader's entire story and then some has been told. Also the threat he once represented is entirely diminished due to his presence in pop culture for almost 50 years. Dude is printed on children's bedsheets and stuff, any excitement from Darth Vader has been wrung entirely dry. Like Bela Lugosi's Dracula, that shit was scary in 1932 but it didn't take long for it to be the way to depixr a vampire in a silly way. If you want a threatening villain to be threatening you don't use them as often as possible cause that means the heroes have to beat or escape them more often, trek did it with the borg as well.

Plus tonally, Vader doesn't fit into Andor at all. Most of Star Wars is focused around grandiose melodrama. Anakin was never relatable at any point. Fucking Mon Mothma at this point has waaaaay more character depth than Darth Vader because what she's going through makes sense and the way she reacts to things emotionally is similar to what a human would do or feel. Anaking never reacted to a single piece of outside stimuli in a way that remotely resembles human behavior. His emotions are dictated by plot necessity at every turn. Andor is doing this thing where it's not trying to be Flash Gordon or whatever bullshit the clone wars was trying to rip off which seems to be the gold standard of star wars for these fucking geeks. A trash kids show.