It's about what the animal is for, though!

So Early Man kills a wild proto-pooch and finds she has a litter. He brings the pups home. Early Woman says "What've you brought those home for?" Early man replies "Feed 'em scraps and in a year or so that's a moon or so's worth of dinners I don't have to go out and hunt!" "All right," Early Woman says "but you're cleaning up after them!" This probably goes on for a few centuries before Not Quite So Early Man works out that he can keep them until they're fully-grown and let them breed before eating them, which keeps a suply of meat handy.

At this point, or somewhat later, there comes a dividng point. Some groups stop here and we get cultures where dogs are for eating. Others discovered, however, that dogs are no end handy when hunting, and pretty good at looking after the kids and nursing mothers when everyone else is out hunting. These became the cultures in which dogs are not eaten but are part of the family (pack).

Cats, on the other hand, were probably considered vermin up until the point somebody realised that they were really good at taking down other vermin. Cats might get larcenous about your meat, but they don't give a damn for your stored grain. What they do do is hunt the rats that are after the grain.

Animals that prove themselves more useful for other things than eating tend to get a free pass.

In the matter of pigs, they are big, strong, fast, very intelligent psychopaths. They are also omnivores who would just as soon eat human as not. So there's no need for qualms of conscience over a ham sandwich.