Because:
Shigeru Miyamoto (the guy who created the Mario and Zelda series) chose Zelda Fitzgerald's name because the exotic sound of her name fit the fantasy game world he created perfectly. He had "the Legend of..." name planned from the start, and used Zelda's name. The main character Link's name was chosen not just because he's the "Link" between the player and the game world (get it?), but because he's the link of all energy through the game's world.
Link is the star of the games, most games begin with details of his life, then he changes due to circumstances that are beyond his control and rises to be a hero, growing not just in spirit but also matures into manhood. What Zelda game ISN'T about Link's life?
Link got a couple games to himself though, Link's Crossbow Training, and Link: The faces of Evil. So, um... Yeah!
In game, the significance of Zelda to the series is she's the mortal reincarnation of the Goddess Hylia who saved the world by hiding the Triforce from the Demon King Demise. After becoming mortal she was able to able to use the Triforce to destroy Demise (even though Link did all the work) and afterward she decided to watch over the Triforce for all time. The royal family is established and each descendant is charged with guarding the Triforce of the Gods. Link has a significant role as the reincarnated spirit of Hylia's chosen hero, and Demise's undying hatred returns again and again as his curse on Link and Zelda. All three are inextricably tied by fate as a never ending cycle (for as long as the games sell) but Zelda is the most central figure because without her none of the stories would happen in the first place.
^^^Yahoo Answers^^^
Why not?