God meets us where we're at. For that matter, he could have explicitly named abortion, thousands of years before it was an issue as well, but He chose not to (at least on the level of the Commandments).
God wants to deal with the root of the sin, much like dealing with weeds- treating the plant itself does nothing long-term, but pulling it up by the weeds does.
What's at the root of slavery? Fear and a desire to control others, greed, etc. What's behind that fear/desire? What was forfeited in the Garden- relationship with a God who has promised to provide all we need (not necessarily want).
Even if certain sins are dealt with, not dealing with the root of sin only results in different pathologies.