Most of the other arguments for jettisoning the old Expanded Universe are arguments I understand, even if I may not fully agree. But this remains the one exception on which I'm totally unconvinced. Most of those people flocking to the new movie didn't know about the existence of the "21 or so years of EU novels." And most of the people flocking to the new movie who did know about the existence of those novels didn't care what was in them. So how can novels be a potential liability to people who don't know or care about them?
Meaning Lucasfilm couldn't justify keeping them going because they couldn't use those stories going forward, nor could they risk more confusion from the new fans the new films might bring in. The ones that might decide, 'Hey there are a whole lot of Star Wars novels, lets see what else happened before this film', and discover, "wait, how come Leia has three kids here and none are named Ben? And what's this war going on, and why is Chewbacca dead?" Those sorts of problems as they go forward. Problems far larger than any Lucasfilm had to deal with when Lucas decided to do something for the PT that was not in line with some of the old EU novels, because most of those cases were minor. This time around it would be major and all levels. Basically everything was different. While they could have attempted to work around it and included everything, or some things, with 21 years of novels going from the Battle of Endor to 40 or so years after that battle, with the new Episode VII happening three-quarters of the way through that time line, they needed to do something before a big mess happened.
I had hoped they'd keep stuff, and to be fair they have, just they are using it differently. The best stuff is getting recycled for use, or a variant is being made to cover it. The thing is, they had to make it somehow clearly different so they could make new materials later that fit what happened between Episode VI and VII one day, and not hamper the writers with 21 years of existing materials (because if they kept it, than the story is already told) They don't need movies with that material, save that the best stuff would not fit the age of the original actors at all, and by the time frame the actors fit their characters, the story is boring, convoluted, and/or would require too much background information to make viable. Information you might not be able to put on screen within the context of the three films and still tell the story. A story that would not have the "mystery box" because all that stuff should be known to the audience already, but wouldn't be because they wouldn't have read the novels that establish what happened to the Empire, the New Repubic, Luke's New Jedi Order, Leia's children, The dozen or so major wars that happened both against Imperial Warlords, and alien races that have reshaped the Republic and Coruscant in particular. The Death of Chewbacca. Luke's child. The tale of his wife. A whole mess of things that would come up in the sequel trilogy, either directly or indirectly via absence. There was too much, so it had to be left behind in order to move forward.
It was not thrown away thought. That is important. Lucasfilm has kept it and while they labeled it "Legends", they have not forgotten about it, nor stopped selling it. They just need a way to keep it separate from the stories they are going to tell that could be the equivalent to an alternate timeline. In one version, Han and Leia didn't get married until about four years after Endor and have three kids over the next three years. In the current version, Han and Leia go married (I think) almost immediately after Endor and had one son roughly a year later. If he has any other siblings, we've not been told about them as of yet (we will assume that Rey is not his much younger sister). IN both versions Luke Skywalker setup an academy of some sort to train new Jedi. In both versions he would have a student fall to the dark side and try to kill the new Jedi. The paths split on the level of success for the betrayal. The "Legends" stories are still there, and some of them might come back in one form or another, but Lucasfilm needed a way to keep the stories straight moving forward with the new Trilogy, and use that opportunity to tie up some older loose ends, or "canonize" one version of a tale told a half dozen times in the various media of the old EU (the Thief of the Death Star Plans is but one extreme example). The next will probably be the Kessel Run and possibly have Han Solo got the Millennium Falcon. There will likely be others as they go forward. Tales that were told serval ways over the decades are usually the fun story everyone want to tell, and with the older versions becoming "Legends" a screenwriter can make up a new version that everyone will see on the big screen rather than on the pages of a novel, or video game, or in a comic book.
"There is always some truth in Legends"
~Ahsoka Tano