This answer will address reasons to keep slavery once it existed, not the reasons to start a slave system in the first place.

Once slavery was entrenched, the major reason to keep it was that it would have cost the slave-owners a tremendous amount to switch systems.  First of all, slave-owners would have had to free their slaves in whom they had a tremendous amount of money invested.  They had bought many of the slaves.  Those that had been born on their own plantations hadn’t been bought, but the owners had paid to clothe and feed them all their lives.  Slave-owners would not likely have been willing to lose their investments by freeing them.  Second, it might have taken a long time to change systems.  The plantation owners would have had to find new workers for the plantations and might have missed out on a growing season or two as they did so.  This, too, would have cost them money.

A secondary reason to keep slavery was it was, in the minds of the Southerners, a good system.  It was, of course, economically good because it extracted labor from the slaves without having to pay for it.  But it was also good, they said, for the slaves.  They said that it was a better system for the slaves because they were taken care of even when they were too young, too sick, or too old to work.  Defenders of the slave system argued that it would be cruel to the slaves to set them free and make them find work in a system like that in which Northern laborers worked.