Besides Ringo, the rest of the slaves are treated almost like furniture or pets in the Sartoris household. With the abuse they get, it's really easy to understand why husband and wife Loosh and Philadelphy decide to escape as soon as they can.
It's not as easy to understand why Louvinia and Joby stay; we have to put on our thinking caps for that one. The answer is probably really complicated, but it might have something to do with the fact that Louvinia raised Ringo and Bayard as though they were her own. Plus, both of them are older and don't know any other home than the Sartoris plantation. Freedom was a frightening prospect, and not everyone was ready to take the plunge into something new.
All the slaves but Ringo are really treated like property; Granny is just as mad about losing them as about losing the silver and the mules. This distance keeps the reader from really getting to know them as complex people.