The books were not fan fictions, they were created as the official continuation of the Star Wars saga, and marketed as such. Many of the major plot points in the series had to be approved by Lucas himself, and some of them were vetoed by him as well.
If anything, the stuff that was there first counts more as canon.
What happened is basically like, if a rich guy (Disney) pruchased the rights to the Shenmue license, and made a Shenmue 3 where Ryo gives up on his quest and goes on a vacation to Hawaii. And since that rich guy now owns the legal rights to Shenmue, he now claims that his new version is the true canon.
If that happened, the outcry from Shenmue fans would be HUGE.
Im sure Lucasfilm and Disney are aware of this and they took the most sensible course of action to please majority of star wars fans.
But the easy way to please EVERYBODY was to make the movies take place in the book timeline. There is this misconception that casual fans would have suddenly needed to read all the books just to understand the movies- this is false. They could have written the movie in a way that no prior knowledge is required, and introduced the existing characters to new audiences through the new movies.
I mean, by that logic, casual fans wouldn't even be able to watch TFA without reading books like "Aftermath", which is part of the new continuity (and sucks btw). People say, "fans don't wanna have to read books just to enjoy the movies", yet Disney is creating the exact same situation with their new books.
It's not a reboot, it's a continuation.
It also reboots 30+ years of post-Endor continuation (100 by including the Legacy comics). It doesn't reboot EVERYTHING, but it reboots the timeline back to the year 4 ABY.
This is like, if Shenmue III the Gang Edition was considered canon to the Shenmue universe despite having nothing to do with the original creators, and then Shenmue III gets announced, and Yu is like,"yeah, any media related to Shenmue III besides the official Shenmue III is totally not canon and we are not going to pay attention to them or consider them as a part of our universe. That should be obvious but..."
No, it's nothing like that. Also, you're creepy and gross, stop stalking me, freak.