That is because it is not necessary. There always will be people who cannot take care of themselves.
IOW: They made slavery of SOME illegal to make slaves of us ALL.
That was the point.
Instead of a war, people who didn't want slavery should have ponied up and bought their freedom. In an act of covetousness to take the property of others, they became property themselves. That is why the Tenth Commandment is there.
Not practical under the economics of the times. I have a good history of Texas that cites a close estimate of the hypothetical value of Texas slave holdings in 1860. It was $160,000,000 -- more than the sum of all other improved property in the State. It was just an insane number, and that was just Texas and their (primarily) sugar cane hands.