Instead of removing a show from the air because of racial inequality, why couldn't they just have a more diverse line-up? This is a popular show like COPS. Why did they cancel Live PD?
Primarily the answer is 'public relations'. The events of the last few weeks (principally the death of George Floyd) have significantly changed the public perception of the police and the Black Lives Matter movement.
Fairly or unfairly it is now an environment where scripted or reality cop shows are less favorably viewed, therefore less profitable.
In addition to this, it has come to light that PD Live destroyed video of an in-custody death. While nothing I have read about this indicates that this wasn't in line with their contract with police, and it was after the investigation into the death was completed - it certainly adds to the negative publicity around the show.
TL;DR Making entertainment shows from live policing does not feel right to broadcasters in the current climate.