Twitter user @PushDustIn recently shared translations of a Famitsu column detailing Sora's arrival into Super Smash Bros. Ultimate. This column states that Sora had an expected arduous barrier of entry to prevail against due to his ties with Disney.
However, a chance meeting at an award show between Sakurai and a Disney executive led to the talks initially beginning. After Sakurai expressed his desire to see Sora join the title, lengthy discussions between Nintendo, Disney, and Square Enix occurred to see this dream finally come to fruition.
This is quite poetic in a sense, considering how the sheer existence of Kingdom Hearts as a franchise only happened because of a chance meeting.
In addition, Sora's internal codename is "Trail," interestingly enough. This perhaps hearkens to the end of the original Kingdom Hearts, where Sora, Donald, and Goofy chase Pluto across the long and winding trail leading to Castle Oblivion, but that's just speculation on my part.