World of Darkness and other Storyteller/Storytelling system games call their GM the Storyteller, which I think is a good title for it.

As for why designers make up their own, the term Game Master is pretty generic, and kinda boring, and they want to create something unique and/or more flavorful to their games. The term "Dungeon Master" is also owned by Hasbro and also only really makes sense in more Tolkien inspired fantasy games where heroes are crawling their way through dungeons.