Star Wars continuity was reset in 2014, when Lucasfilm declared the old Expanded Universe non-canon (or "Legends"), but George Lucas' Star Wars: The Clone Wars - which launched in 2008 - was really the beginning of this. For most of the Star Wars franchise’s history, there was only one main continuity: The Expanded Universe, which included the six saga films, several spinoff films and TV shows, and a plethora of video games, comic books, novels, and other properties. Where there was conflict, it was resolved with a "tiered" approach in which the Expanded Universe adapted to fit around Lucas' latest movies; continuity problems were often retconned out of existence.
All that changed in 2014, two years after Disney's acquisition of Lucasfilm. Disney officially declared the Expanded Universe an alternate continuity (dubbed "Legends") for the sake of the Star Wars sequel trilogy, allowing them to maximize their creativity with a mostly-clean slate. Surprisingly, though, this decision was foreshadowed by George Lucas and Dave Filoni's Star Wars: The Clone Wars in 2008, which set the pattern in motion.
George Lucas' Clone Wars Set A New Approach For Canon & Continuity
The Clone Wars era of the Legends continuity was covered thoroughly from 2002 through 2007 via the Clone Wars multimedia project, which included, among other things, numerous issues of the Star Wars: Republic comics, seven novels, and the Star Wars: Clone Wars animated micro series by Genndy Tartakovsky. Lucas and Filoni's Star Wars: The Clone Wars, despite its positive reception among Star Wars viewers, contradicted the Clone Wars multimedia project at every turn. The series rewrote the timeline of Anakin Skywalker’s Jedi Knighthood, gave him a padawan (Ahsoka Tano), resurrected Darth Maul and Eeth Koth, and significantly changed character like Asajj Ventress and Barriss Offee, among many other discrepancies.
With its many retcons to the established Clone Wars era of the Expanded Universe, it was clear that The Clone Wars could not fit into the Legends timeline, even with a few Legends properties attempting to reconcile these discrepancies. Dave Filoni addressed this in 2012 via the 134th issue of Star Wars Insider, where he explained that he did not consider The Clone Wars part of the Expanded Universe. As he explained:
"There is no more clear illustration of the difference between the Expanded Universe and the Star Wars created by George Lucas. The EU is a well of ideas, and there's what's on screen. They don't live in the same universe. Everyone wants to think so, I know, and there is a lot of effort to make it all work, but it's pretty clear when you really start looking at it that when you take ideas from the printed realm and bring it on the screen, it's not always the same. They relate. There are similarities... They inspire you, which I think is the whole point of having an EU. We try hard to honor things where we can, to give nods to things, but at the end of the day there is a difference between what you see in the Star Wars films and TV series and what you see in those books."
The only Star Wars properties that Filoni considered canonical while making The Clone Wars were the saga films, which explains why there were countless contradictions (with the Mandalorians a particularly notable example). Filoni would often use the Expanded Universe stories as loose inspiration so he could do his own thing, and he considered that "the whole point of having an EU." This is exactly how the current canon continuity uses Legends.
Disney Followed George Lucas' Lead After The Clone Wars
Filoni's attitude toward the Expanded Universe set the pattern for Disney's approach. Although Lucas had approved all materials in the original Star Wars Expanded Universe, he either inspired or even ideated many changes made in The Clone Wars - notably Darth Maul's survival and the drastically reimagined Witches of Dathomir (he once told Filoni that "continuity is for wimps," demonstrating his own attitude). Disney simply followed Lucas' lead after The Clone Wars, making it all official.
It's interesting to note that Filoni's habit of disregarding canon printed media in favor of a loose adaptation has continued in the Disney era. Star Wars: Tales of the Jedi presented a very different of Ahsoka's return to the rebellion to the one seen in E.K. Johnston's novel Ahsoka, for example, and Star Wars: The Bad Batch season 1, episode 1 surprised viewers by showing how Kanan Jarrus survived Order 66 - a tale already told in comics. Filoni now runs Lucasfilm Television (and is working on a Star Wars movie), and he clearly hasn't changed his approach at all.
Star Wars' Canon Reset Was Inevitable (Even If Lucas Hadn't Sold It To Disney)
George Lucas’ plans for the Star Wars sequel trilogy were consistent with his newfound attitude towards continuity; his proposed storylines completely disregarded established Expanded Universe lore (not least in ignoring Luke Skywalker's marriage to Mara Jade, a character he disliked). This, combined with the havoc The Clone Wars wreaked on the Legends timeline, meant a continuity reset was frankly inevitable - even if Disney hadn't purchased Lucasfilm. The difference is that Lucas likely would not have gone to the trouble of rebranding the EU as Legends, thereby establishing it as a legitimate, albeit discontinued, continuity. He'd probably have just pressed on with his movies, leaving viewers and Lucasfilm creatives with a messy continuity to clean up.
As it stands, the Expanded Universe is a fairly consistent and tidy timeline, as long as one disregards The Clone Wars. The Legends rebranding, while frustrating to some longtime Star Wars viewers, confirms that the now-bygone timeline was once the official Star Wars canon and effectively saved it from a far confusing state, had Lucas remained in charge of Lucasfilm and made the Star Wars sequel trilogy. Star Wars: The Clone Wars, meanwhile, should be considered the first exclusive property to Disney’s new continuity, as it was never meant to belong to the Legends timeline in the first place.