Answer:
Because they still needed people to tend to the crop industry, and everyone else had gone to war or was coming back.
They really had no reason to free them.
Throw aside the moral compass for a minute and go back in time. If you had somebody working for you, and did all the stuff you needed them to do, and you didn't have to pay them, wouldn't you keep them around? There was no punishment for keeping them, and they got a bunch of work done without any additional cost.
