Think about it. The first movie was basically the Hero's Journey IN SPACE, and it was well-enough made that lots of people liked it. It kicked around for a while, surviving the Christmas Special (if you don't know about the Star Wars Christmas Special, lucky you) and the conflicts in creating the next two movies. After it was all over, people did what comes naturally to them when they see something good: they start writing fan fiction. Then, someone gets their fanfiction published. "Hey!" says a random aspiring writer. "I liked Star Wars! This might be just as good!" So he reads it, decides he can do one better, and writes more, gets it published, presumably by the same desperate publishing house, and eventually the Thrawn trilogy happens (translated: a really influential series of Star Wars books that kicked off the Expanded Universe proper) and people started making all sorts of new stories in the Star Wars universe.

Every writer being separate and yet everything being linked up meant that people could make their own factions, build on each other's work, construct social systems based on terms the movies had only thrown out to avoid some minor plot problem, etcetera. Even the prequels couldn't dampen their style, because now they could just make new stories in the far-flung past about the early days of the Galactic Republic.

I'm not going to claim I understand the Star Wars EU as it is, because it's complex, convoluted, and has so many loose ends it's frayed on all the edges. But, Star Wars was never really my beat anyway.

Fast forward to decades in the future, and Star Wars has the largest and most well-known Expanded Universe of basically any franchise, with hordes of fans working endlessly to assemble everything into a coherent timeline and a working picture of affairs in their galaxy. People can get involved at any point in time, write stories for any of its probably-pushing-a-thousand species and multiplicity of worlds, and build a little fortress on a sea of much cooler fortresses in the hope that someone else will come along and say "Oh, I like the architecture of your fortress. I think I might take a few cues from you when building mine."

And then, J. J. Abrams happened. Yeeeaaahh. Not gonna pass judgement on that until I see what comes out, but the guy hasn't really shown proper respect for established canon and common sense before, and so I can be forgiven for bracing myself rather than waiting excitedly. I know that the new movies wouldn't actually be about the Expanded Universe, because it's still fanfiction and not even Disney would make a movie based on a series of fan works, but some people would be likely to work in the gaps, or make something that causes only a fraction of the older stories to be mutually exclusive to the new timeline. J. J. Abrams does not strike me as one of those people. At all.