Once upon a time in the old Tubeworld Dynamic website, I speculated on the reason why Superman didn't come to the rescue of the TV version of the World Trade Center on September 11th, 2001. The Toobworld splainin was that Superman was already dead in the TV Universe, having died from Krypton radiation poisoning in the early sixties. (This was after he rescued a couple of characters from the TV series 'Crime Story' from an atomic test site.)
But Russell T. Davies gave us a TV concept that would have meant the Twin Towers were doomed even if Superman had been alive: fixed points in Time.
Here's a splainin from the Doctor Who Wiki:
The Laws of Time were connected to the concept of "fixed points" in time -- events and/or individuals who have such a long-standing impact on the timeline that no one, not even Time Lords, were allowed to interfere with their natural progression. The Doctor, while free to interfere in alien invasions and save planets in most cases, cannot interfere/interact with these fixed points. Examples include Jack Harkness after his rejuvenation by Rose Tyler (DW: Utopia (although the Doctor nonetheless shared several adventures with him), the destruction of Pompeii by the Vesuvius volcano (DW: The Fires of Pompeii, and the death of explorer Adelaide Brooke (DW: The Waters of Mars).
The case of Brooke marks one of the only times the Doctor has intentionally interfered with a fixed point, under the rationale that, being the last surviving Time Lord, the Laws of Time were now his to command. Ultimately, he is unable to prevent Brooke from committing suicide, thereby allowing the timeline to unfold, with only a few minor changes. (DW: The Waters of Mars) However, the 2009 Dalek Invasion of Earth, despite the fact it led to Brooke becoming a fixed point in time, was not, itself, a fixed point and therefore the Doctor was able to stop it from continuing. (DW: Journey's End).
The resolution of the Last Great Time War, which involves the destruction of Gallifrey, has been "time locked", rendering it technically an unalterable -- and inaccessible -- point in space and time. Nonetheless, Dalek Caan was able to circumvent the restriction and retrieve Davros, though at the cost of his sanity. (DW: The Stolen Earth/Journey's End)
The Time Lords themselves once sanctioned what could be the greatest-ever attempt at interfering with a fixed point in time, when they assigned the Doctor the task of preventing the creation of the Daleks. The Laws of Time, however, remained intact as the Doctor did not do so. (DW: Genesis of the Daleks)
Exactly what constitutes a fixed point in time is unclear. For example, the birth of Earth religious leader Jesus Christ was a major event in the cultural evolution of the planet Earth, so much so its common calender was dated from his estimated year of birth, and innumerable aspects of human civilization related to worship of the man. Yet the Doctor once claimed to have influenced the birthplace of Jesus, who was born in a barn due to the inn in Bethlehem having no rooms; the Doctor said he'd gotten the last room at the inn. (DW: Voyage of the Damned). However, much like the eruptions of Vesuvius, it may have been simply a case of the Doctor experiencing a predestination paradox and being part of the natural progression of events.
Makes your head hurt, don't it?
So this is why there's no fantasy rewind for events like the collapse of the Twin Towers, the explosion of the Challenger space shuttle, the sinking of the Titanic, the assassinations of Lincoln, the Kennedys, Gandhi, and Dr. King.....