Here's a thing people often get wrong. The old novels, comics etc. that are now under the 'Legends' banner were never actually "canon" at all. When George Lucas still owned the company, the only "canon" was the stories he was involved with, which is basically just the (only, at the time) two movie trilogies and The Clone Wars (the CG one, not the Genndy Tartakovsky one that came before.)
Literally everything else was apocryphal, and every author, artist and game developer that created the material knew that going in, and have said so repeatedly. They weren't adding to the canon, they were supplementing it. They were playing with George's toys, but they were still George's toys at the end of the day. Sometimes a piece of art or design from the EU would strike a chord with George and he may pluck it out and incorporate it in some way, but it would typically be it's own thing within canon.
The idea that the EU/Legends works were somehow one coherent "canon" that's been supressed is both asinine and divorced from reality. It was a hodgepodge of a thousand different stories that was riddled with inconsistencies, contradictions and retcons on top of retcons on top of retcons on top of retcons on top of retcons.
So yeah, when Disney took over they wanted some kind of order and coherence to the media they intended to sell, so rather than continue an old product line that was not so much on it's last legs as it's last worn down stumps, they opted to clear the decks and at least try to keep anything new going forward all part of the same canon as the six movies and one TV show that had already laid the foundation. Of course with the realities of producing all of this, there's already been a handful of contradictions, but nothing major, and the depth and meaningfulness of the cross-pollination between media has been unprecedented.
So who decides what's canon? The owners of the intellectual property, because that's literally what that means.
