Slaves were seen as property. When a slave ran away it was seen as 'stealing', even if the slave were the one doing the 'stealing'. Many slaves ran away because their family had been taken from them, their 'masters' were cruel, and often because they were tired of being treated as if they were not human and worthy of respect.
Because they couldnt put up with the abuse! Duh. If you were a slave, would you put up with the abuse of working hard and being fed tablescraps?