The Left localize it as a crime unique to Western,white Europeans because then it becomes useful to attack and smear the West.  They do do not care about slavery per se…they only care about how they can further denigrate Western civilization.  One of the main tactics is to uniquely vilify Western civilization for immoral attributes that are shared across just about every other civilization(such as slavery), even though Western Europe has probably done more to break the chains of slavery across the world than any other place in history.