Stability due to international cooperation and the decline of Communism across the world.
A lord of the manor could not sell his serfs<span> as a Roman might sell his </span>slaves<span>. On the other hand, if he chose to dispose of a parcel of land, the </span>serfs<span> associated with that land stayed with it to serve their new lord; simply speaking, they were implicitly sold in mass and as a part of a lot.</span>