The word "canon" has changed quite a bit over the years and has many meanings now, one of which is authentic work by the original author (or chose successor)
That is the definition we use must use when describing fictional stories.
Do you seriously need to ask why?
Why is the person who made the story the only person we should trust as having the actual story in mind?
I dunno, maybe because they made the story in the first place.
It's theirs. Not anyone else's. Anyone else's alterations do not matter, the original is the only authentic one in regards to the story.