But that's because their publishers and Lucasfilm's licensing people overtly claimed they were canon, something Pocket and Paramount/CBS never did with Trek books (except with Mosaic and Pathways for a while, even after Voyager started contradicting them).
The thing is, they weren't entirely wrong to say that, even if it was misleading. A canon is a unified body of works considered complete or authoritative -- not only in a story-continuity sense, but in a wider sense of something like the Shakespeare canon, the film noir canon, etc. So since the SW tie-ins did strive to maintain a consistent continuity among themselves, they could be said to constitute a canon -- just not the canon of the movies (and later TV series). Similarly to how you could say the Marvel Cinematic Universe constitutes a canon of its own even though it's distinct from the Marvel Comics canon it adapts. And eventually Lucasfilm and the publishers did start defining "canon levels," with the tie-ins as a secondary tier of canon less authoritative than the screen productions.
The problem is that fandom has gotten the idea that canon means "correct" or "real" rather than just "comprehensive." They see it as a seal of approval or "truth," as if there were such a thing as a "true" version of an imaginary universe. So they don't grasp how a franchise can potentially have more than one canon within it. The misperception of the word as a value judgment or official sanction, that competitive "there can be only one" mentality, prohibits using it that way without misunderstanding.