Then there is the fact that the people who consider the Star Trek novels to be canon are some of the most fanatically devoted trekkers
out there - they'd have to be to have read them all. There are hundreds of Star Trek novels which are in no way designed to work together or with the various series (with the exception of Peter David
's books). When you try to establish them in some sort of order things get kinda...crowded. Saying the books are 'unofficial' allows us all to have a starting point to work from - most fans know the series, many less have read the books. If the novels were considered canon any moderate-level fan could be trumped in an argument by "yeah, but in the Next Generation novel "The Children of Hamlin
And then there's the fact that most of the novels are really, really bad
. The first few dozen Next-Gen ones were passable, but after that...ick.
I go for movies, series and select paper references (like the enterprise-D Technical Manual) as canon. the rest of it is essentially fan-fiction. The Star Trek universe is already insanely complicated, the last thing we need is more information for the fans to keep straight.