prosfilaes
Adventurer
1. You claim that it takes movies, but then state that it came about with Sherlock Holmes ... which wasn't a movie.
Actually, it has been many movies, and the first ones before Doyle was finished writing. But the first canon in this sense was Holmes, even if the concept was stronger and more broadly influential with shared universes over multiple mediums.
2. Many people attribute the first use of "fan canon" to a parody essay written by someone mocking ... Biblical canon studies.
And the name "the Big Bang" was to make fun of the idea. It doesn't change how serious the idea is.
So, yeah, a very new idea when it comes to artistic creation.
It's much older than RPGs, quite a bit older than video games, about as old as movies, and only slightly younger than comic strips. True, like opera and the novel, it's a newcomer compared to poetry and painting.
As for why it matters? Because the idea of "canon" and a "shared universe" that isn't simply the whim of a single artist (pace Tolkein) but is, rather, a corporate commodity (IP) that allows for many creators and is "policed" by fans is a very recent concept.
But why does it matter? As you say, it goes hand in hand with the recent concept of a "shared universe". It's not a problem if a concept is a new one to explain a new phenomenon.
It's kind of strage that, inter alia, decision made by a random person on the set of Star Trek in the 1960s to save money is elevated by fans into canon, and then forced back by those fans onto the owner of the property.
Fans can't force anything; they can just choose not to spend money on something they don't like. It seems weird to blame the people who support you for wanting something; I seriously doubt any property not as well loved as Star Trek could have survived such mistakes as the first movie.
Does it make you unhappy that a movie called Macbeth, based off the work of Shakespeare, had better hew closely to the play of Shakespeare's, even though the one extant copy is questionably accurate to Shakespeare's original? If you're selling your product based on a name, you've got to live up to the expectations of that name.
The same things that people enjoy about canon are also the same things that cause the things they don't like (retcons, reboots, and so on).
Those are about continuity.