Because the professional police force was first established to maintain slave society in the South. Then it was extended to maintain wage slavery in the industrial centers. Then when private slavery was abolished they were retooled to re-enslave freed black people by selectively enforcing new laws targeting behaviors of freed people (prison population went from 95% white to 95% black in a decade). Then they were extended to enforce the convict leasing program where the black people that the cops arrested were leased to former slave owners to make profits for the government. Then they were extended to take orders from factory owners to break strikes up to and including murdering unarmed workers. Then they were extended when local cops wouldn't kill their neighbors, so the state police were built based on the US occupying force in The Philippines.

They have always been this way. They have never stopped being this way. They cannot be reformed. They must be fully demolished and a net new structure must be rebuilt that supercedes the few legitimate parts of their role in society.

This will never happen under capitalism.