The 31-year-old director, who was hired a year ago to helm the second in a series of planned standalone movies for Lucasfilm and Disney, said it was "the hardest decision," according to Hollywood Reporter.
"It was the hardest decision I've ever had to make in my life," the director said.
"I knew that this was going to be questioned and it was going to come under scepticism as to why I left Star Wars," Trank elaborated.
Although an official statement framed his departure as a "personal decision", problems during the shoot for Trank's upcoming reboot of Fantastic Four for Fox and his "erratic" behavior were said to have elicited pause among Lucasfilm executives.