Walking the Ridgeway with Joshua.”So Daddy, why can’t the Doctor just go back in time and stop that Dalek from shooting him?”
“Because he can’t travel back over his own timeline. Just not done. Ever.”
“I see.”
“Besides, if he hadn’t been shot, he wouldn’t have had the near-regeneration that eventually created the second Doctor. Donna would then have died in the fire when the TARDIS fell into the core. And they wouldn’t have saved the Earth.”
“That’s right!”
“So that’s why – ”
“And then there would have just been a blank screen and a lot of screaming and wailing as everyone in the whole universe got killed.”
“…Yes.”
“Because he can’t travel back over his own timeline. Just not done. Ever.”
“I see.”
“Besides, if he hadn’t been shot, he wouldn’t have had the near-regeneration that eventually created the second Doctor. Donna would then have died in the fire when the TARDIS fell into the core. And they wouldn’t have saved the Earth.”
“That’s right!”
“So that’s why – ”
“And then there would have just been a blank screen and a lot of screaming and wailing as everyone in the whole universe got killed.”
“…Yes.”
“WOTAN!”
Brian of Morbius 