Bishop73

Chosen answer: Because just like in the comic books, all super heroes and villains have names that are a representation of either the abilities they possess, paraphernalia they use, their personalities, and/or the circumstances of their origin. Magneto is a much more compelling and memorable name for a character than simply calling him Erik. Presumably it also avoids confusion if there are several mutants with the same "regular" name, to have a unique nickname for each one.

Phaneron ★

Answer: In addition to what was said, the nicknames also provide the individual mutant a way to identity more closely with the fact that they are mutants. As Mystique said in "X-Men: The Last Stand" when addressed as Raven, "I don't answer to my slave name." Many mutants are rejected by their parents when they find out they're mutants, so the "nickname" serves as a way to distance themselves from those that rejected them. In addition, at times their nicknames are called "code names." In this case, it would be a way to address a mutant without using their real identity, in essence a secret identity that's common in almost all superheroes and super villains.

Bishop73

Answer: No definitive answer has been given, although director Bryan Singer has said a possible explanation is Magneto helps. At the end of the Wolverine film, Professor X and Magneto are waiting for Wolverine at the airport. So at some time in the future, Magneto could have rebonded the metal to Wolverine's bone (which has happened in the comics). In the comics though, the True Adamantium turned into Beta Adamantium due to the presence of Logan's mutant healing factor gene. Beta Adamantium has regenerative powers as well (it's also what allows bones to function as they grow properly) and Logan's metal claws could have possibly just grown back, over time.

Bishop73

Answer: Bryan Singer didn't like the idea of using Juggernaut again, since Brett Ratner used him in The Last Stand. He felt it wasn't original enough. He was inspired by watching high speed videos online and did a re-shoot using Quicksilver.

Bishop73