I'm not an American but have lived there in the past for many years. It has always baffled me how Americans are willing to blame the left or the right instead of the system as a whole. Maybe because if they did so, they would be undermining the very foundations that their country was built on.
for example: https://www.justice.gov/opa/pr/justice-department-announces-...
Who is President is very much matters. You clearly see this in the Reagan/Bush/Clinton years, where mass incarceration was in vogue, at the direction of the Attorney General (William Barr), who is part of the executive branch, leading the charge.
I think there's a lot more to it than that. There's also the matter of a lot of war vets becoming police officers, the approach to policing they learned in the military, and their lasting effect on police department culture.