Although he didn’t part ways with the show on the best terms, Chris has said that he’s excited for the chance to play the character again. “It will be exciting to revisit the Ninth Doctor’s world,” he said in a statement, “bringing back to life a character I love playing.”
The ninth doctor was the first in the show’s “modern era,” and kicked off a series that is still releasing new episodes. Since Christopher left the role, it’s been taken up by David Tennant, Matt Smith, and Jodie Whittaker, among others. Many previous doctors have jumped at the chance to reprise their roles in other formats, but this is the first time that Christopher has chosen to do so for understandable reasons.
Source : https://www.distractify.com/p/why-christopher-eccleston-left-doctor-who