The fact that it has no officially defined canon? There's no Gene Roddenberry, George Lucas, J. Michael Straczynski, or Joss Whedon proclaiming what counts and what doesn't.
There usually doesn't have to be. "Canon" is just a nickname for the original body of work, as distinguished from derivative works like tie-ins and fan fiction. The only thing that ever really needs to be defined is how the tie-ins relate to it. Usually they are separate from the canon by default, but there are occasional exceptions (increasingly often these days), and that's when you need things defined.
Roddenberry never actually
needed to define what was canon. It just bruised his ego when people called Diane Duane "the creator of the Rihannsu," so he felt the need to make a statement claiming ownership. And while he was at it, he decided to say the animated series didn't count, partly because its ownership was up in the air at the time, but (I suspect) partly because he'd fallen out with D.C. Fontana and wanted to devalue her contributions to the franchise. But that had nothing to do with what the makers of actual canon were doing, since Roddenberry's involvement with TNG by that point was strictly honorary. The actual producers were just making the show, which was canon by definition, and they had no problem referencing TAS when it suited them, or ignoring Diane Duane's books when it suited them, or whatever. Creators define canon by what they do, not by some official declaration.
Similarly, the "proclamations" of Lucasfilm only applied to the status of the tie-ins relative to the films and to each other. That was subject to change, but the status of the films themselves was a given. Although it gets iffy in the case of
Star Wars because you've got TV movies and shows whose relationship to the canon has been defined differently at different times (e.g. the Ewok movies, the
Ewoks and
Droids cartoons, and the first
Clone Wars cartoon).
In the case of JMS and Whedon, I don't think they ever needed to "declare" anything canon. Canon in its simplest sense means what the creator of the property creates. So when the creators make their own tie-ins, or at least closely supervise them, those tie-ins can be canonical in a way that other people's tie-ins usually can't, because the creators are too busy making the main show to ride close enough herd on the work of the people doing the tie-ins. JMS wanted all the B5 novels to be canonical, but he wasn't able to supervise the first run of books closely enough because he was still making the show, so inconsistencies got through. It wasn't until the show was over that he was able to devote enough attention to the later line of tie-ins to make them canonical. So it wasn't a matter of declaration so much as just the way things worked out. Same with Whedon -- he couldn't supervise the
Buffy and
Angel tie-ins that came out during the shows as closely as he could the ones that came after the shows. Or at least, the ones that came out after the shows didn't run the risk of being contradicted by new episodes.