I have a bit more of a radical view of this...
Copyright has become a tool to enforce privatization of culture, which is a pretty modern take—and, obviously, it's just another example of the ongoing attempts to squeeze every last possible dollar out of everything and everyone. Culture is supposed to belong to us, it's supposed to be our culture.
Another way of looking at this is that absurd choice of word, "canon." This is a religious term, for what a particular religious establishment accepts as the Truth, from among all the spiritual and theological writings a particular religious tradition has produced. But why does the Pope have the right to choose which stories are True? And why does Disney have the right to choose which stories are True?
This is one of the subtler reasons that Star Trek is so much better: while there is still a Star Trek Canon, that canon was massively diversified by their spec script policy (back in the days that Star Trek was still worth watching).
Anyway, this is also why fanfiction is such a big thing.