It's important to remember that Lucas made Star Wars canon up as he went along (major example of that in the movies themselves here). Lucas himself even made things that are no longer canon (and that many would prefer to forget). But the general gist (even before Disney) was that only the movies were actually canon (LucasArts fan relations circa 2001, emphasis mine)
When it comes to absolute canon, the real story of Star Wars, you must turn to the films themselves - and only the films. Even novelizations are interpretations of the film, and while they are largely true to George Lucas' vision (he works quite closely with the novel authors), the method in which they are written does allow for some minor differences. The novelizations are written concurrently with the film's production, so variations in detail do creep in from time to time. Nonetheless, they should be regarded as very accurate depictions of the fictional Star Wars movies.
The further one branches away from the movies, the more
interpretation and speculation come into play. LucasBooks works
diligently to keep the continuing Star Wars expanded universe cohesive
and uniform, but stylistically, there is always room for variation.
Not all artists draw Luke Skywalker the same way. Not all writers
define the character in the same fashion. The particular attributes of
individual media also come into play. A comic book interpretation of
an event will likely have less dialogue or different pacing than a
novel version. A video game has to take an interactive approach that
favors gameplay. So too must card and roleplaying games ascribe
certain characteristics to characters and events in order to make them
The analogy is that every piece of published Star Wars fiction is a
window into the 'real' Star Wars universe. Some windows are a bit
foggier than others. Some are decidedly abstract. But each contains a
nugget of truth to them. Like the great Jedi Knight Obi-Wan Kenobi
said, 'many of the truths we cling to depend greatly on our point of
(–LucasBooks' Chris Cerasi as quoted by Steve Sansweet's answer to the question "I'm really confused about canon. Is Star Wars Gamer canon? What about the Marvel series? Are they now considered "Infinities?", Ask the Lucasfilm Jedi Council)
So Disney didn't just jump out here with something new. LucasArts didn't consider any of the books hard canon, so Disney was free to explicitly reject them as such.