In the Doctor Who two-parter Time of Angels/Flesh and Stone, a race called Aplans is mentioned. The Aplans were said to have been wiped by the Weeping Angels in the maze. When the Doctor threw them all into the crack why didn't the Aplans come back to life? When the Angels fell in they were unwritten from time and ceased to exist so they were not there to kill the Aplans.
In the Doctor Who universe, existence seems to be quite lazy when it comes to correcting paradox, changing as little as possible to maintain normalcy.
Example: Amelia's parents fall into the crack, if they don't exist, how does she? Removing her, and thus undo all of the things she has done would be messy, so she just never realized they disappeared, but she lives on.
So basically to avoid the butterfly effect, the universe fudges things that have a casual link with something being impacted (wibbly wobbly and all that).
This event was caused by a collapse of what is known as the Time Field. The collapse of the Time Field occurs when the TARDIS explodes in the year 2010. The collapse of the Time Field was distributed unevenly and would appear randomly at different points throughout space-time.
The nature of the temporal energies from the crack in the wall allowed for certain kinds of individuals to be removed from existence without altering the events they had been responsible for. Hence Amy's parents disappeared from space-time but Amy continued to exist. Being near the center of the temporal event and bathed in time energy made Amy, unwittingly, a temporal center and paradox agent.
The Time Field was a field of time energy that spilled from cracks in the fabric of time and space. The cracks were caused by the Doctor's TARDIS exploding on 26 June 2010, after coming under the control of an unknown entity who was somehow connected to the Silence and whose voice was heard repeating the words "Silence will fall". Tardis Data Core Wiki > Time Field
Since the explosion was the TARDIS in the year 2010, it caused temporal ripples which altered space-time, upended causality and creating paradox events which allowed events to exist without evidence of cause.
In the case of the Angels, they attacked and killed the Aplans in the past, but when the Angels engaged the crack attempting to absorb its temporal energy, they were both erased and the temporal fissure was closed removing them from the timestream. However, the Aplans who were at the center of the temporal event (dead, as it were) were not altered.
While this is a paradox, it is consistent with paradox events where a creature at the center of a paradox event remembers the event but are unchanged by it. In this case, the Aplans were dead and remained such even though the Angels were removed from the event which supposedly killed them.
It is highly possible the Universe when it corrected the paradox, had the Aplans die from a virulent disease or some other cataclysmic event which would lead to the same overall state, the Aplans died and were buried in the catacombs.
Other cracks released pure time energy able to wipe individuals from time itself and remove events from history. Time travellers such as the Doctor would still remember them, at least so long as the removed event or person did not relate to the time traveller's direct past. (TV: Flesh and Stone, Cold Blood)
Also, when the time energy erased people and events, the consequences would still remain: the Byzantium remained crashed when the Weeping Angels who had caused it were erased; and Amelia Pond still existed when her parents were erased from history. (TV: Flesh and Stone, The Big Bang) This time energy was visible as a glow of bright white light, which sometimes extended tendrils from the crack towards nearby people and objects to consume them.
The Doctor guessed that the time energy from these kind of cracks had erased events such as the CyberKing walking over London in the Victorian era, and the 2009 Dalek invasion of Earth, the latter being one of Earth's most publicly visible invasions. This explained why Amy Pond did not remember those events. Smaller, inconsequential things were erased as well, including, possibly, the ducks from a duck pond in Leadworth. (TV: The Eleventh Hour, Flesh and Stone) Tardis Data Core Wiki > Time Field
It is stated in the series that the farther away things are from the crack, the less they are affected (or something to that effect). The crack is not only in space, but also in time. Also, the crack starts as a small crack that can be relatively harmless, or even a means of travel, to something like we saw on the Byzantium. If the Aplans were wiped out before the crack became the "all consuming" crack it was in their time, the effect of the Angels ceasing to exist must not have reached them (almost as if they only ceased to exist since the crack became active). I know this theory doesn't make a whole lot of sense, but like a lot of things in the Doctor's Universe, most things aren't thought out very well before they are written into the story. I mean, they turned the Statue of Liberty into an Angel... I think they just gave up trying to have it all work together and make sense.
Think of the cracks as paradox generators; or better yet, think of the universe as a carpet. The cracks tear individual threads from that carpet, but although a few threads are missing, the whole thing still holds together despite some missing pieces. It may not make sense to think about how one thread is connecting to the other with the one in between them missing, but it's a small enough inconsistency not to matter all that much until too many threads are ripped out and the holes become more noticeable (and the carpet becomes less stable).
Now apply that logic to the cracks. They rip things out of space and time, and although what they rip out never existed, everything that happened because of that thing still happened. It doesn't make sense how it happened without the missing piece, but most people will just overlook it, as seen with the cleric who didn't remember his fellow soldiers and Amy who simply "lost" her parents. The cracks work in such a way that while they do erase individual things from history, the consequences and impact of that thing's existence stays. That is why the Byzantium remained crashed and why the Aplans remained dead even though the Weeping Angels were erased, why Amy still existed despite her parents being erased, and why Amy still had Rory's engagement ring and a picture of him despite him being erased. Even though the Angels, Amy's parents, and Rory were erased, all of the actions they took in life were still there and still happened.