Very briefly:
Back in the day in small rural communities a few nice white kids could get drunk and lynch a nigger and the sheriff would buy them a round.
Some people didn't like this arrangement for whatever reason.
They couldn't get things done at the state level through law enforcement reform because the state legislatures happen to be full of those old sheriffs.
So they went to DC, where the guys in power are rich noblesse oblige sorts. After a series of debates these folks decided that the best way to handle it was to send a fucking swarm of suitjacketed sunglassed feds to wherever a dark-skinned corpse floated up, with the intention of having them stare the local police down into action and, if not able, to completely wreck their shit and take over the matter themselves and send the perpetrators to federal PMITA prison. Doing so required some sort of crime, so they made killing black people for being black people a federal crime.
As time goes on, similar problems pop up in areas where there's one group with a near monopoly on political power who doesn't much like another group in the same area to the extent of some individuals randomly killing others. The laws have been extended to cover these.