Its a rather odd name, doncha think, who, in good conscience, would name their kid Adolf?
Der Fuhrer sorta spoiled it for future generations.
Why has Adolf Hitler's first name become unpopular but not Josef Mengele's, Elizabeth Báthory's, Charles Manson's, Mary Ann Cotton's, etc?
I doubt that Adolf is unpopular in German-speaking countries; it's just one of those names that hasn't really transitioned into English.
Given that the Western world tends to associate the name with a monster it's unlikely to become popular.
The other names you mentioned are in common usage and are not associated primarily with one person.'
Adolf stands out more than any of those others so it's easier for it to become stigmatized since few people were named it anyway.
One of my penfriends, when I was still capable of writing in French, was Adolphe. He was born, and named after the second war, so the name is not THAT hated.
Because there are so many other people with those other names that have done well. St. Joseph, or Joseph Pulitzer. Queen Elizabeth, or Elizabeth Barrett Browning. Charles Darwin, or Charles Dickens. Mother Mary, Mary Queen of Scots.
How many other Adolfs do you know of? There is Adolf Eichmann, but he was also a Nazi, so that isn't going to help give the name something positive.