I don't think most people really care about animals suffering, or not. However, most people do care about themselves and their children.
Long to short, factor farming generates a lot of ground level pollution. Furthermore, those animals also generate a lot of greenhouse gases. Frame the problem in terms of human suffering and more people are likely to pay attention.
Years ago there was a NY Times Sunday magazine feature article on factor pig farming. It was enlightening. Unfortunately, I've searched for it and can't seem to find it. If you can find, do read it.
p.s. Growing crops to be fed to farm animals is also very inefficient. More people would have more food if we all ate less animal protein.