Well I read the manga once, but that was a long time ago so I can't really say anything about why it's like that in AgK, however normally the title of a series refers to the most important aspect of the entire series. This does not necessarily have to refer to the main character. For instance One Piece refers to the treasure in fact the entire OPverse is searching for, including the main characters. It's their main drive. Even though it's still completely unknown what it is and actually only gets sporadically mentioned, it's so important for the entire story you can name the series after it. Fairy Tail is named after a mage guild and everything that happens in the story is always directly connected to the guild. Etc.
It can be a risky business though:
Zelda fan: I love the Legend of Zelda, best game ever.
Random Person: Yes I love Zelda and his cute green Elven outfit.
Zelda fan: Elven outfit? You mean Link?
Random Person: Who the fudge is Link?
*Zelda fan brutally murders Random Person*
Titles can be deceiving. There's still an entire discussion going around where Bleach is supposed to come from. I personally always preferred the idea of it referring to the cleaning or 'bleaching' of human souls, the job of the Shinigami. However there are people who say this was never confirmed and is something made up and consequently they provide alternative reasonings.
