Because she kind of is central to the lore of the Zelda universe She's the incarnation of the goddess Hylia. You are her warrior of light who reincarnates time and time again when the world needs a hero to push back darkness. She is very much the central character and all the "why's" almost always revolve around her. Link might be the main protagonist and the player controlled character, but Zelda is the soul of the story. Goddess Hylia for the record is the legend, a story passed down the ages in that series.

People seriously still debate this? Nintendo answers this question for you if you play the freaking games.

Why is it called Resident Evil when the threat went global?

Link isn't the main character. He's not even a character.

They couldn't have named it "The Misadventures of Ganon, the Unlucky". So they just took the name of the nearest chick.

In the orginal game the booklet and the intro of the game says that after Ganon steals the triforce of power Zelda splits the triforce of wisdom into small pieces and hides it all over Hyrule do prevent Ganon from aquiring it. She then tasks her handmaid, Impa, to find a hero to collect the triforce of wisdom and defeat Ganon. After this Zelda is captured by Ganon.

An interpetation you can do about this story is that Impa wanders the lands for many years telling the story of Zelda in her search for a hero. And thus this becomes an old woman going from village to village, hamlet to hamlet, telling a lengend about princess and a hidden power.

My interpetation makes the title fit the story from a 1986 perspective without later installments and the contrived goddess Hylia BS.

The name just sounds good. Catchy name equals more sales.

Link is the main character, because he is the full (developed) character you play as. The name of the game doesn't matter.

It's dumb to suggest that the person or thing named in the title should be what you play as.

Well they were really hesitant to establish the character of "Link" early on... otherwise, I just think of it as being akin to the etiquette for how plays were often named in the past, they being named after the highest ranking figure in play even when they didn't play a particularly large role :D

If Shakespeare had lived in Hyrule he too would have been naming his plays after Zelda rather than Link.

Since, I've already given a serious reply:

Why are they called the Mario Brothers when one of them has the first name Mario? There is nothing super about that.
Why is it called Dragon Quest, when you play a human? Shouldn't it be called human quest?
Why is he called Pac-Man when he isn't going away anywhere?
How can they call it Metal Gear and not have any Metallica?

And lastly,

Why is it called the XBox 1, when it is the third console? It's like they can't even count at Microsoft. Zero, 360, 1,... Hey man, that's just not how numbers work!

1. "Luigi" is Italian for "other Mario"

2. Dragon is related to the quest itself, i.e. the goal, and not the undertaker of that quest. And the goal of the quest always has to do with dragons.

3. Lame pun

4. that doesn't even make any sense

5. You answered that yourself. MS just doesn't know how numbers work. Stop shaming people with severe dyscalculia!

He is in BotW. Link is offially his name now.