According to a Deadline, David Benioff and DB Weiss have officially exited their upcoming Star Wars trilogy and according to Deadline.
In a joint statement to Deadline, They said "We love Star Wars when George Lucas built it, He built us. It was the thrill of a lifetime and we will always be indebted to the saga that changed everything".
Kathleen Kennedy also made a statement, "We left the door open for the creators to come back. David, Benioff, and Dan. Weiss are incredible storytellers. We hope to include them in the journey forward when they are able to step away from their busy schedules to focus on Star Wars".
So it seems like mainly the reason that they stepped away as per Deadlines. It was timing and their commitment to other projects. In August, the duo signed a pretty massive deal with Netflix to develop a huge slate of original projects for that streaming platform.