Because it is a story about the Weeping Angels and not the Reapers.

the Doctor said afterwards that they were simply lucky.

Before the Reapers could get there all the energy from the paradox was drained erasing Winter Quay

Also note that the Reapers only appeared because the paradox created in "Father's Day" was worsened due to the fact that there were two Ninth Doctor and Rose's present at the exact same time.