The difference is in the scale of the slaughter and the actual slaughtering being dissociated from the eating for many of these animals, such that a disproportionate minority takes on the task of killing for a majority of meat consumption. This has recorded known harmful effects on the people doing the killing as well as the community in which the killing is done, in a way other large industrial processes do not.

As an aside: this is a gotcha question and doesn't fundamentally address the position that animal slaughter is cruel. It relies on an obviously false rhetoric that something performed by non-sapient creatures in an environment where the only way to survive for some creatures is largely through the personal killing of other creatures is morally equivalent to an environment where sapient creatures who have wide dietary availability make the choice to consume the flesh of another creature that has been usually killed for them.