People in poverty generally have shorter lifespans. In the U.S., African Americans are disproportionately situated in impoverished, low-income inner cities so they face a greater number of harmful environmental challenges compared to, say, someone raised in an affluent neighborhood.
Poor people have to deal with environmental hazards like lead poisoning (Flint, Michigan ring any bells?), a lack of affordable and high quality healthcare, the unhealthy consumption of processed foods and sugary drinks (cardiovascular diseases are a pressing health issue among African Americans), and the threat of recurring criminal violence (a symptom of poverty).
There are a plethora of harmful stressors that are quite specific to African Americans, which is due mostly in part to their unique history here in the U.S.; it's not that they are biologically predisposed to have a shorter life expectancy compared to other populations.