The situation was too extreme for them. The Reapers appeared in "Father's Day" when Rose caused a small, very local wound in time. In "The Wedding of River Song", it wasn't a small wound -- the whole of time was coming apart at the seams, throughout the entire universe.
River had prevented a fixed point in time from happening, that is why the universe collapsed. Rose created a paradox by preventing her father from dying. It's possible that Pete's death was not a fixed point as the Doctor suggested Rose's change to the timeline could remain changed. Furthermore, Rose's paradox in "Father's Day" was worsened because there were two Ninth Doctor's and Rose's present in the same area.