You have something...a movie, a book, whatever...that’s really good. So good that it demands sequels and spin-offs. This “expanded universe” then fills in all kind of additional information about the elements of the original work. Every thing in the “expanded universe” ends up hanging off those original elements until they become these epic things they were never meant to be.
And so, the “expanded universe”—with all its back-references to the original work—ends up feeling pretty small. By making even the ordinary elements of the original into the epic and iconic, it seems to even tarnish what made the original special as well.
I don’t really mean to say that such an “expanded universe” is a mistake. It’s just an observation. It would be nice to see an effort to counteract that trend.