I think one of the old-EU explanations was that the Emperor purposely didn’t allow Vader to be fully healed so that he would always carry the pain and scars of his failure in order to better keep him under his heel.
I could see that. He’s completely servile to the Emperor in the originals. However, it really takes away from the character if he has the ability to regain his old appearance, but his boss is like, “No, pretty much destroyed the Jedi, ensured my domination of the galaxy, but you failed to kill Obi-Wan. I’m withholding plastic surgery from you.” I’d like to think the reason he holds a grudge towards Obi-Wan, is because he suffered irreparable damage from him, turning him into a monster. Even if his wife had still been alive, he’s disfigured and trapped in a walking respiratory device.