Is it a matter of becoming calloused of necessity, as George Elliot suggested when she said that if we could notice everything around us we could hear the squirrel's heartbeat and the grass growing, and we should die of the noise? We can't emotionally afford to react fully to everything.
Is it because so much of animal suffering is inflicted on total innocence? So many of the awful things that happen to people are caused by people--like wars and genocide. But what about the random horrors of natural disasters? You can sit in a movie theater and weep in the darkness while the faithful dog starves on his master's grave, and watch TV news with your jaw clenched, but in control, while images of starving children fill the screen.
Way back in Aristotle's day, thinking people learned about catharsis. Is it somehow easier to experience it at one remove?
Oh, the book is The Postmistress by Sarah Blake.