We're breaking the pattern a little here, since before now the Who Elements have been focused on characters that aren't your MC. The Chosen One, however, is almost always the main character. Rarely (but always with very interesting results) the Chosen One is your main antagonist.
There are a few reasons for this, and I'll do my best to outline them below.
Reason #1: The Chosen One is Special
This may seem like a super obvious statement, but let's dissect it a little. Why does it matter that the Chosen One is special, and what does that have to do with the Chosen One being the Main Character?
Reason #1.1: You're Supposed to Empathize with the Main Character
If you can't empathize with the main character, then your main character is not one your readers will be terribly interested in.
Reason #1.2: People Like Being Special
The Main Character is most often the one that your readers are meant to identify with, and what better way to make them feel special (which they want to) than by making the character they identify with most undeniably special?
I don't know about you, but after I read Harry Potter and the Sorcerer's Stone as a kid, I waited for MONTHS to get my acceptance letter from Hogwarts.
Reason #2: The Chosen One is (Usually) Central to the Plot
While it is possible to write a Main Character that is not the Chosen One, but is rather a companion to or antagonist for the Chosen One, it's not very easy, because by default (not always, but as a rule of thumb) the story revolves around the Chosen One because of their Chosen-ness. Being asked to identify with the witness to the story rather than the cause/effect of the story can put some readers off. (Take a look at stories like The Great Gatsby or The Adventures of Sherlock Holmes for reference, where the narrator is a character in the story but the story isn't about them.)
There may be other reasons that the Chosen One is usually the main character, but these strike me as the main two. It would be an interesting challenge to write a story wherein the main character was not the Chosen One but wanted to be - like if Wyld Style had been the main character of the Lego Movie, or if Callum from The Dragon Prince remained an everyman while the others had their special Choosey things.
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