In case of TL&DR, here's a synopsis:
Many people can be rescued, but aren't because when the Doctor discovers that they have been sent into the past, he also finds that they have lived full lives that the Doctor is unwilling to undo. (Not to mention that his knowledge of it makes it much harder for him to do so.) He would, in effect, be killing them, and creating another version. This was hard enough in a more direct case with The Girl Who Waited, but her life was at least miserable; the other Weeping Angel victims we've seen had good lives, even if they were out of their correct eras.
In Amy & Rory's case, however, they were in a Time with serious Temporal Disturbance, created a bigger one by being the heart of a Paradox, and then went right back and lived from the time they had fixed; they were simply WAY to complicated for the Tardis to be able to extract without another massive rupture of Time like we saw with The Doctor when he was not 'killed.'
The Doctor begged Amy not to allow herself to be caught by the angel because it would create a fixed point in time, preventing him from saving her.
They never really explore the nature of Fixed Points, but there are some consistent things we've seen about them, specifically that they tend to BY THEMSELVES influence many people and have a significant impact on the universe.
That being said, time seems resistant to change, becoming more and more so as you approach something that is a Fixed Point. Minor things are simple to change, but don't really have a significant effect, as they become larger and larger, however, the resistance increases, and a truly fixed point can damage the timeline itself.
- Adelaide Brooke's death was a fixed point, but had enough flexibility in terms of long-term effects that minor changes would still result in the same major outcome, with some small changes in the immediate results. (Where she died, the newspaper, etc -- but the effect on Reality was the same.)
- Rose's interference in things had the potential to become larger... and the Reapers appeared, to sterilize it. Again, restoring the net effect, although allowing a small change in the immediate time.
- The Doctor's 'Death' was too huge, and there was no easy way around it; more to the point, River Song 'killing' him was the fixed point. It's influence was fairly far ranging, AND intersects the Doctor's own time line later at multiple points. Eliminate the 'killing' and you eliminate much of the backstory we've already seen have effects on him.
Now, the Weeping Angels themselves created a massive Temporal influence, as we see when the Doctor has so much trouble trying to land the Tardis in the first place. Generating a Paradox to eliminate them, as Amy & Rory did, created even more Temporal Turbulence; still worse, Amy & Rory have gone back to a time that THEY THEMSELVES have deleted from history.
It's never explained exactly how such things work, but, basically, it looks like despite the apparent current placidity of the era where they ended up, on the 'backside' of time, that is a nasty snarl.
Could the Doctor wait a few years and save them? He could try, but it probably wouldn't work; after enough has passed, or if they move far enough away, their influence would be reduced, but they have been influencing the time they are in, and the universe has slowly been correcting itself around their influence. This wouldn't be a problem for most people, but they were mildly 'Complicated Space-time events' courtesy of being Time-Travelers, and the whole Paradox to stop the angels has made them MUCH more complicated Space-Time events; you can think of it as giving them massive inertia.
All that being said, I have no real backup; this seems consistent with what they've shown, and the Doctor's habits when it comes to changing his own / his companion's timeline, but I still wouldn't put it past them to find a way to at least bring Amy back in a season or two.
