With the return of Russell T Davies for a new era of Doctor Who, fans of this 60-year-old storytelling universe are revisiting their favorite Doctors from eras past. Immediately following the first four seasons of Davies' first run on Doctor Who, Steven Moffat took over to shape the modern show around the next incarnations of the character. However, despite having the same showrunner, both the Eleventh and Twelfth Doctors were developed very differently from one another. While fans have many theories, there may be a strong narrative reason for this difference. Ever since it was decided to swap out the Doctor's original actor, William Hartnell, for Patrick Troughton, the concept of regeneration is a bit hard to wrap one's head around.
Source：https://www.cbr.com/doctor-who-eleventh-twelfth-doctors-difference/
