Well, the first two "THIS BOOK 'MATTERS'!" Babylon 5 novels were also the only two reprinted by the second publisher to pick up the license. So they obviously thought that there was a bigger demand for those two than for the other seven novels.
Except that's not quite true. We were initially told that all of the Dell B5 novels would be treated as canonical. That was Straczynski's intent. But as it turned out, his duties running the TV show didn't allow him to oversee the novels as closely as he wanted, and so they were wildly uneven in terms of their compatibility with canon, to the point that JMS ended up treating only #7 and #9 as part of the canon (and he's said that #7 is only about 90% canonical). Since JMS directly oversaw the Del Rey B5 novels and even outlined them himself, I would assume the reason only Dell #7 & 9 were reprinted is because they were the only ones he wanted to be reprinted.
So all the Dell books were supposed to "matter" to the canon, but only two of them ended up doing so. It wasn't feasible for the books to become an integral part of the series canon until the series was no longer in production and JMS was free to oversee the books directly.
And it's easier in a case where a series has one singular creator, like JMS for B5 or Joss Whedon for Buffy. In those cases, if that creator oversees books or comics and calls them canonical, then that's what they are -- assuming, of course, that the creator doesn't decide otherwise when he gets the chance to do more screen material (as George Lucas did). In the case of Star Trek, where the creators of canonical material come and go and can have different opinions of what's canonical, it becomes an even more futile exercise. Jeri Taylor treated her VGR novels as canonical, but her successors did not. As Greg said, any declaration of canonicity is not going to hold up for a second if a future filmmaker decides he or she has a better idea.
Not to mention that canons contradict themselves all the time. Review just about any long-running TV or movie series and you'll find things in the early installments that are retconned or contradicted later on, sometimes subtly, sometimes blatantly. The priority of storytellers is to make the story they're telling now as good as possible, and they usually won't throw out a good idea because it conflicts with something they did years ago.
So it makes zero real-world difference whether you stick the "canon" label on something or not. It won't make it any less likely to be contradicted.